Based on kernel version 4.3. Page generated on 2015-11-02 12:51 EST.
1 2 Timekeeping Virtualization for X86-Based Architectures 3 4 Zachary Amsden <email@example.com> 5 Copyright (c) 2010, Red Hat. All rights reserved. 6 7 1) Overview 8 2) Timing Devices 9 3) TSC Hardware 10 4) Virtualization Problems 11 12 ========================================================================= 13 14 1) Overview 15 16 One of the most complicated parts of the X86 platform, and specifically, 17 the virtualization of this platform is the plethora of timing devices available 18 and the complexity of emulating those devices. In addition, virtualization of 19 time introduces a new set of challenges because it introduces a multiplexed 20 division of time beyond the control of the guest CPU. 21 22 First, we will describe the various timekeeping hardware available, then 23 present some of the problems which arise and solutions available, giving 24 specific recommendations for certain classes of KVM guests. 25 26 The purpose of this document is to collect data and information relevant to 27 timekeeping which may be difficult to find elsewhere, specifically, 28 information relevant to KVM and hardware-based virtualization. 29 30 ========================================================================= 31 32 2) Timing Devices 33 34 First we discuss the basic hardware devices available. TSC and the related 35 KVM clock are special enough to warrant a full exposition and are described in 36 the following section. 37 38 2.1) i8254 - PIT 39 40 One of the first timer devices available is the programmable interrupt timer, 41 or PIT. The PIT has a fixed frequency 1.193182 MHz base clock and three 42 channels which can be programmed to deliver periodic or one-shot interrupts. 43 These three channels can be configured in different modes and have individual 44 counters. Channel 1 and 2 were not available for general use in the original 45 IBM PC, and historically were connected to control RAM refresh and the PC 46 speaker. Now the PIT is typically integrated as part of an emulated chipset 47 and a separate physical PIT is not used. 48 49 The PIT uses I/O ports 0x40 - 0x43. Access to the 16-bit counters is done 50 using single or multiple byte access to the I/O ports. There are 6 modes 51 available, but not all modes are available to all timers, as only timer 2 52 has a connected gate input, required for modes 1 and 5. The gate line is 53 controlled by port 61h, bit 0, as illustrated in the following diagram. 54 55 -------------- ---------------- 56 | | | | 57 | 1.1932 MHz |---------->| CLOCK OUT | ---------> IRQ 0 58 | Clock | | | | 59 -------------- | +->| GATE TIMER 0 | 60 | ---------------- 61 | 62 | ---------------- 63 | | | 64 |------>| CLOCK OUT | ---------> 66.3 KHZ DRAM 65 | | | (aka /dev/null) 66 | +->| GATE TIMER 1 | 67 | ---------------- 68 | 69 | ---------------- 70 | | | 71 |------>| CLOCK OUT | ---------> Port 61h, bit 5 72 | | | 73 Port 61h, bit 0 ---------->| GATE TIMER 2 | \_.---- ____ 74 ---------------- _| )--|LPF|---Speaker 75 / *---- \___/ 76 Port 61h, bit 1 -----------------------------------/ 77 78 The timer modes are now described. 79 80 Mode 0: Single Timeout. This is a one-shot software timeout that counts down 81 when the gate is high (always true for timers 0 and 1). When the count 82 reaches zero, the output goes high. 83 84 Mode 1: Triggered One-shot. The output is initially set high. When the gate 85 line is set high, a countdown is initiated (which does not stop if the gate is 86 lowered), during which the output is set low. When the count reaches zero, 87 the output goes high. 88 89 Mode 2: Rate Generator. The output is initially set high. When the countdown 90 reaches 1, the output goes low for one count and then returns high. The value 91 is reloaded and the countdown automatically resumes. If the gate line goes 92 low, the count is halted. If the output is low when the gate is lowered, the 93 output automatically goes high (this only affects timer 2). 94 95 Mode 3: Square Wave. This generates a high / low square wave. The count 96 determines the length of the pulse, which alternates between high and low 97 when zero is reached. The count only proceeds when gate is high and is 98 automatically reloaded on reaching zero. The count is decremented twice at 99 each clock to generate a full high / low cycle at the full periodic rate. 100 If the count is even, the clock remains high for N/2 counts and low for N/2 101 counts; if the clock is odd, the clock is high for (N+1)/2 counts and low 102 for (N-1)/2 counts. Only even values are latched by the counter, so odd 103 values are not observed when reading. This is the intended mode for timer 2, 104 which generates sine-like tones by low-pass filtering the square wave output. 105 106 Mode 4: Software Strobe. After programming this mode and loading the counter, 107 the output remains high until the counter reaches zero. Then the output 108 goes low for 1 clock cycle and returns high. The counter is not reloaded. 109 Counting only occurs when gate is high. 110 111 Mode 5: Hardware Strobe. After programming and loading the counter, the 112 output remains high. When the gate is raised, a countdown is initiated 113 (which does not stop if the gate is lowered). When the counter reaches zero, 114 the output goes low for 1 clock cycle and then returns high. The counter is 115 not reloaded. 116 117 In addition to normal binary counting, the PIT supports BCD counting. The 118 command port, 0x43 is used to set the counter and mode for each of the three 119 timers. 120 121 PIT commands, issued to port 0x43, using the following bit encoding: 122 123 Bit 7-4: Command (See table below) 124 Bit 3-1: Mode (000 = Mode 0, 101 = Mode 5, 11X = undefined) 125 Bit 0 : Binary (0) / BCD (1) 126 127 Command table: 128 129 0000 - Latch Timer 0 count for port 0x40 130 sample and hold the count to be read in port 0x40; 131 additional commands ignored until counter is read; 132 mode bits ignored. 133 134 0001 - Set Timer 0 LSB mode for port 0x40 135 set timer to read LSB only and force MSB to zero; 136 mode bits set timer mode 137 138 0010 - Set Timer 0 MSB mode for port 0x40 139 set timer to read MSB only and force LSB to zero; 140 mode bits set timer mode 141 142 0011 - Set Timer 0 16-bit mode for port 0x40 143 set timer to read / write LSB first, then MSB; 144 mode bits set timer mode 145 146 0100 - Latch Timer 1 count for port 0x41 - as described above 147 0101 - Set Timer 1 LSB mode for port 0x41 - as described above 148 0110 - Set Timer 1 MSB mode for port 0x41 - as described above 149 0111 - Set Timer 1 16-bit mode for port 0x41 - as described above 150 151 1000 - Latch Timer 2 count for port 0x42 - as described above 152 1001 - Set Timer 2 LSB mode for port 0x42 - as described above 153 1010 - Set Timer 2 MSB mode for port 0x42 - as described above 154 1011 - Set Timer 2 16-bit mode for port 0x42 as described above 155 156 1101 - General counter latch 157 Latch combination of counters into corresponding ports 158 Bit 3 = Counter 2 159 Bit 2 = Counter 1 160 Bit 1 = Counter 0 161 Bit 0 = Unused 162 163 1110 - Latch timer status 164 Latch combination of counter mode into corresponding ports 165 Bit 3 = Counter 2 166 Bit 2 = Counter 1 167 Bit 1 = Counter 0 168 169 The output of ports 0x40-0x42 following this command will be: 170 171 Bit 7 = Output pin 172 Bit 6 = Count loaded (0 if timer has expired) 173 Bit 5-4 = Read / Write mode 174 01 = MSB only 175 10 = LSB only 176 11 = LSB / MSB (16-bit) 177 Bit 3-1 = Mode 178 Bit 0 = Binary (0) / BCD mode (1) 179 180 2.2) RTC 181 182 The second device which was available in the original PC was the MC146818 real 183 time clock. The original device is now obsolete, and usually emulated by the 184 system chipset, sometimes by an HPET and some frankenstein IRQ routing. 185 186 The RTC is accessed through CMOS variables, which uses an index register to 187 control which bytes are read. Since there is only one index register, read 188 of the CMOS and read of the RTC require lock protection (in addition, it is 189 dangerous to allow userspace utilities such as hwclock to have direct RTC 190 access, as they could corrupt kernel reads and writes of CMOS memory). 191 192 The RTC generates an interrupt which is usually routed to IRQ 8. The interrupt 193 can function as a periodic timer, an additional once a day alarm, and can issue 194 interrupts after an update of the CMOS registers by the MC146818 is complete. 195 The type of interrupt is signalled in the RTC status registers. 196 197 The RTC will update the current time fields by battery power even while the 198 system is off. The current time fields should not be read while an update is 199 in progress, as indicated in the status register. 200 201 The clock uses a 32.768kHz crystal, so bits 6-4 of register A should be 202 programmed to a 32kHz divider if the RTC is to count seconds. 203 204 This is the RAM map originally used for the RTC/CMOS: 205 206 Location Size Description 207 ------------------------------------------ 208 00h byte Current second (BCD) 209 01h byte Seconds alarm (BCD) 210 02h byte Current minute (BCD) 211 03h byte Minutes alarm (BCD) 212 04h byte Current hour (BCD) 213 05h byte Hours alarm (BCD) 214 06h byte Current day of week (BCD) 215 07h byte Current day of month (BCD) 216 08h byte Current month (BCD) 217 09h byte Current year (BCD) 218 0Ah byte Register A 219 bit 7 = Update in progress 220 bit 6-4 = Divider for clock 221 000 = 4.194 MHz 222 001 = 1.049 MHz 223 010 = 32 kHz 224 10X = test modes 225 110 = reset / disable 226 111 = reset / disable 227 bit 3-0 = Rate selection for periodic interrupt 228 000 = periodic timer disabled 229 001 = 3.90625 uS 230 010 = 7.8125 uS 231 011 = .122070 mS 232 100 = .244141 mS 233 ... 234 1101 = 125 mS 235 1110 = 250 mS 236 1111 = 500 mS 237 0Bh byte Register B 238 bit 7 = Run (0) / Halt (1) 239 bit 6 = Periodic interrupt enable 240 bit 5 = Alarm interrupt enable 241 bit 4 = Update-ended interrupt enable 242 bit 3 = Square wave interrupt enable 243 bit 2 = BCD calendar (0) / Binary (1) 244 bit 1 = 12-hour mode (0) / 24-hour mode (1) 245 bit 0 = 0 (DST off) / 1 (DST enabled) 246 OCh byte Register C (read only) 247 bit 7 = interrupt request flag (IRQF) 248 bit 6 = periodic interrupt flag (PF) 249 bit 5 = alarm interrupt flag (AF) 250 bit 4 = update interrupt flag (UF) 251 bit 3-0 = reserved 252 ODh byte Register D (read only) 253 bit 7 = RTC has power 254 bit 6-0 = reserved 255 32h byte Current century BCD (*) 256 (*) location vendor specific and now determined from ACPI global tables 257 258 2.3) APIC 259 260 On Pentium and later processors, an on-board timer is available to each CPU 261 as part of the Advanced Programmable Interrupt Controller. The APIC is 262 accessed through memory-mapped registers and provides interrupt service to each 263 CPU, used for IPIs and local timer interrupts. 264 265 Although in theory the APIC is a safe and stable source for local interrupts, 266 in practice, many bugs and glitches have occurred due to the special nature of 267 the APIC CPU-local memory-mapped hardware. Beware that CPU errata may affect 268 the use of the APIC and that workarounds may be required. In addition, some of 269 these workarounds pose unique constraints for virtualization - requiring either 270 extra overhead incurred from extra reads of memory-mapped I/O or additional 271 functionality that may be more computationally expensive to implement. 272 273 Since the APIC is documented quite well in the Intel and AMD manuals, we will 274 avoid repetition of the detail here. It should be pointed out that the APIC 275 timer is programmed through the LVT (local vector timer) register, is capable 276 of one-shot or periodic operation, and is based on the bus clock divided down 277 by the programmable divider register. 278 279 2.4) HPET 280 281 HPET is quite complex, and was originally intended to replace the PIT / RTC 282 support of the X86 PC. It remains to be seen whether that will be the case, as 283 the de facto standard of PC hardware is to emulate these older devices. Some 284 systems designated as legacy free may support only the HPET as a hardware timer 285 device. 286 287 The HPET spec is rather loose and vague, requiring at least 3 hardware timers, 288 but allowing implementation freedom to support many more. It also imposes no 289 fixed rate on the timer frequency, but does impose some extremal values on 290 frequency, error and slew. 291 292 In general, the HPET is recommended as a high precision (compared to PIT /RTC) 293 time source which is independent of local variation (as there is only one HPET 294 in any given system). The HPET is also memory-mapped, and its presence is 295 indicated through ACPI tables by the BIOS. 296 297 Detailed specification of the HPET is beyond the current scope of this 298 document, as it is also very well documented elsewhere. 299 300 2.5) Offboard Timers 301 302 Several cards, both proprietary (watchdog boards) and commonplace (e1000) have 303 timing chips built into the cards which may have registers which are accessible 304 to kernel or user drivers. To the author's knowledge, using these to generate 305 a clocksource for a Linux or other kernel has not yet been attempted and is in 306 general frowned upon as not playing by the agreed rules of the game. Such a 307 timer device would require additional support to be virtualized properly and is 308 not considered important at this time as no known operating system does this. 309 310 ========================================================================= 311 312 3) TSC Hardware 313 314 The TSC or time stamp counter is relatively simple in theory; it counts 315 instruction cycles issued by the processor, which can be used as a measure of 316 time. In practice, due to a number of problems, it is the most complicated 317 timekeeping device to use. 318 319 The TSC is represented internally as a 64-bit MSR which can be read with the 320 RDMSR, RDTSC, or RDTSCP (when available) instructions. In the past, hardware 321 limitations made it possible to write the TSC, but generally on old hardware it 322 was only possible to write the low 32-bits of the 64-bit counter, and the upper 323 32-bits of the counter were cleared. Now, however, on Intel processors family 324 0Fh, for models 3, 4 and 6, and family 06h, models e and f, this restriction 325 has been lifted and all 64-bits are writable. On AMD systems, the ability to 326 write the TSC MSR is not an architectural guarantee. 327 328 The TSC is accessible from CPL-0 and conditionally, for CPL > 0 software by 329 means of the CR4.TSD bit, which when enabled, disables CPL > 0 TSC access. 330 331 Some vendors have implemented an additional instruction, RDTSCP, which returns 332 atomically not just the TSC, but an indicator which corresponds to the 333 processor number. This can be used to index into an array of TSC variables to 334 determine offset information in SMP systems where TSCs are not synchronized. 335 The presence of this instruction must be determined by consulting CPUID feature 336 bits. 337 338 Both VMX and SVM provide extension fields in the virtualization hardware which 339 allows the guest visible TSC to be offset by a constant. Newer implementations 340 promise to allow the TSC to additionally be scaled, but this hardware is not 341 yet widely available. 342 343 3.1) TSC synchronization 344 345 The TSC is a CPU-local clock in most implementations. This means, on SMP 346 platforms, the TSCs of different CPUs may start at different times depending 347 on when the CPUs are powered on. Generally, CPUs on the same die will share 348 the same clock, however, this is not always the case. 349 350 The BIOS may attempt to resynchronize the TSCs during the poweron process and 351 the operating system or other system software may attempt to do this as well. 352 Several hardware limitations make the problem worse - if it is not possible to 353 write the full 64-bits of the TSC, it may be impossible to match the TSC in 354 newly arriving CPUs to that of the rest of the system, resulting in 355 unsynchronized TSCs. This may be done by BIOS or system software, but in 356 practice, getting a perfectly synchronized TSC will not be possible unless all 357 values are read from the same clock, which generally only is possible on single 358 socket systems or those with special hardware support. 359 360 3.2) TSC and CPU hotplug 361 362 As touched on already, CPUs which arrive later than the boot time of the system 363 may not have a TSC value that is synchronized with the rest of the system. 364 Either system software, BIOS, or SMM code may actually try to establish the TSC 365 to a value matching the rest of the system, but a perfect match is usually not 366 a guarantee. This can have the effect of bringing a system from a state where 367 TSC is synchronized back to a state where TSC synchronization flaws, however 368 small, may be exposed to the OS and any virtualization environment. 369 370 3.3) TSC and multi-socket / NUMA 371 372 Multi-socket systems, especially large multi-socket systems are likely to have 373 individual clocksources rather than a single, universally distributed clock. 374 Since these clocks are driven by different crystals, they will not have 375 perfectly matched frequency, and temperature and electrical variations will 376 cause the CPU clocks, and thus the TSCs to drift over time. Depending on the 377 exact clock and bus design, the drift may or may not be fixed in absolute 378 error, and may accumulate over time. 379 380 In addition, very large systems may deliberately slew the clocks of individual 381 cores. This technique, known as spread-spectrum clocking, reduces EMI at the 382 clock frequency and harmonics of it, which may be required to pass FCC 383 standards for telecommunications and computer equipment. 384 385 It is recommended not to trust the TSCs to remain synchronized on NUMA or 386 multiple socket systems for these reasons. 387 388 3.4) TSC and C-states 389 390 C-states, or idling states of the processor, especially C1E and deeper sleep 391 states may be problematic for TSC as well. The TSC may stop advancing in such 392 a state, resulting in a TSC which is behind that of other CPUs when execution 393 is resumed. Such CPUs must be detected and flagged by the operating system 394 based on CPU and chipset identifications. 395 396 The TSC in such a case may be corrected by catching it up to a known external 397 clocksource. 398 399 3.5) TSC frequency change / P-states 400 401 To make things slightly more interesting, some CPUs may change frequency. They 402 may or may not run the TSC at the same rate, and because the frequency change 403 may be staggered or slewed, at some points in time, the TSC rate may not be 404 known other than falling within a range of values. In this case, the TSC will 405 not be a stable time source, and must be calibrated against a known, stable, 406 external clock to be a usable source of time. 407 408 Whether the TSC runs at a constant rate or scales with the P-state is model 409 dependent and must be determined by inspecting CPUID, chipset or vendor 410 specific MSR fields. 411 412 In addition, some vendors have known bugs where the P-state is actually 413 compensated for properly during normal operation, but when the processor is 414 inactive, the P-state may be raised temporarily to service cache misses from 415 other processors. In such cases, the TSC on halted CPUs could advance faster 416 than that of non-halted processors. AMD Turion processors are known to have 417 this problem. 418 419 3.6) TSC and STPCLK / T-states 420 421 External signals given to the processor may also have the effect of stopping 422 the TSC. This is typically done for thermal emergency power control to prevent 423 an overheating condition, and typically, there is no way to detect that this 424 condition has happened. 425 426 3.7) TSC virtualization - VMX 427 428 VMX provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP 429 instructions, which is enough for full virtualization of TSC in any manner. In 430 addition, VMX allows passing through the host TSC plus an additional TSC_OFFSET 431 field specified in the VMCS. Special instructions must be used to read and 432 write the VMCS field. 433 434 3.8) TSC virtualization - SVM 435 436 SVM provides conditional trapping of RDTSC, RDMSR, WRMSR and RDTSCP 437 instructions, which is enough for full virtualization of TSC in any manner. In 438 addition, SVM allows passing through the host TSC plus an additional offset 439 field specified in the SVM control block. 440 441 3.9) TSC feature bits in Linux 442 443 In summary, there is no way to guarantee the TSC remains in perfect 444 synchronization unless it is explicitly guaranteed by the architecture. Even 445 if so, the TSCs in multi-sockets or NUMA systems may still run independently 446 despite being locally consistent. 447 448 The following feature bits are used by Linux to signal various TSC attributes, 449 but they can only be taken to be meaningful for UP or single node systems. 450 451 X86_FEATURE_TSC : The TSC is available in hardware 452 X86_FEATURE_RDTSCP : The RDTSCP instruction is available 453 X86_FEATURE_CONSTANT_TSC : The TSC rate is unchanged with P-states 454 X86_FEATURE_NONSTOP_TSC : The TSC does not stop in C-states 455 X86_FEATURE_TSC_RELIABLE : TSC sync checks are skipped (VMware) 456 457 4) Virtualization Problems 458 459 Timekeeping is especially problematic for virtualization because a number of 460 challenges arise. The most obvious problem is that time is now shared between 461 the host and, potentially, a number of virtual machines. Thus the virtual 462 operating system does not run with 100% usage of the CPU, despite the fact that 463 it may very well make that assumption. It may expect it to remain true to very 464 exacting bounds when interrupt sources are disabled, but in reality only its 465 virtual interrupt sources are disabled, and the machine may still be preempted 466 at any time. This causes problems as the passage of real time, the injection 467 of machine interrupts and the associated clock sources are no longer completely 468 synchronized with real time. 469 470 This same problem can occur on native hardware to a degree, as SMM mode may 471 steal cycles from the naturally on X86 systems when SMM mode is used by the 472 BIOS, but not in such an extreme fashion. However, the fact that SMM mode may 473 cause similar problems to virtualization makes it a good justification for 474 solving many of these problems on bare metal. 475 476 4.1) Interrupt clocking 477 478 One of the most immediate problems that occurs with legacy operating systems 479 is that the system timekeeping routines are often designed to keep track of 480 time by counting periodic interrupts. These interrupts may come from the PIT 481 or the RTC, but the problem is the same: the host virtualization engine may not 482 be able to deliver the proper number of interrupts per second, and so guest 483 time may fall behind. This is especially problematic if a high interrupt rate 484 is selected, such as 1000 HZ, which is unfortunately the default for many Linux 485 guests. 486 487 There are three approaches to solving this problem; first, it may be possible 488 to simply ignore it. Guests which have a separate time source for tracking 489 'wall clock' or 'real time' may not need any adjustment of their interrupts to 490 maintain proper time. If this is not sufficient, it may be necessary to inject 491 additional interrupts into the guest in order to increase the effective 492 interrupt rate. This approach leads to complications in extreme conditions, 493 where host load or guest lag is too much to compensate for, and thus another 494 solution to the problem has risen: the guest may need to become aware of lost 495 ticks and compensate for them internally. Although promising in theory, the 496 implementation of this policy in Linux has been extremely error prone, and a 497 number of buggy variants of lost tick compensation are distributed across 498 commonly used Linux systems. 499 500 Windows uses periodic RTC clocking as a means of keeping time internally, and 501 thus requires interrupt slewing to keep proper time. It does use a low enough 502 rate (ed: is it 18.2 Hz?) however that it has not yet been a problem in 503 practice. 504 505 4.2) TSC sampling and serialization 506 507 As the highest precision time source available, the cycle counter of the CPU 508 has aroused much interest from developers. As explained above, this timer has 509 many problems unique to its nature as a local, potentially unstable and 510 potentially unsynchronized source. One issue which is not unique to the TSC, 511 but is highlighted because of its very precise nature is sampling delay. By 512 definition, the counter, once read is already old. However, it is also 513 possible for the counter to be read ahead of the actual use of the result. 514 This is a consequence of the superscalar execution of the instruction stream, 515 which may execute instructions out of order. Such execution is called 516 non-serialized. Forcing serialized execution is necessary for precise 517 measurement with the TSC, and requires a serializing instruction, such as CPUID 518 or an MSR read. 519 520 Since CPUID may actually be virtualized by a trap and emulate mechanism, this 521 serialization can pose a performance issue for hardware virtualization. An 522 accurate time stamp counter reading may therefore not always be available, and 523 it may be necessary for an implementation to guard against "backwards" reads of 524 the TSC as seen from other CPUs, even in an otherwise perfectly synchronized 525 system. 526 527 4.3) Timespec aliasing 528 529 Additionally, this lack of serialization from the TSC poses another challenge 530 when using results of the TSC when measured against another time source. As 531 the TSC is much higher precision, many possible values of the TSC may be read 532 while another clock is still expressing the same value. 533 534 That is, you may read (T,T+10) while external clock C maintains the same value. 535 Due to non-serialized reads, you may actually end up with a range which 536 fluctuates - from (T-1.. T+10). Thus, any time calculated from a TSC, but 537 calibrated against an external value may have a range of valid values. 538 Re-calibrating this computation may actually cause time, as computed after the 539 calibration, to go backwards, compared with time computed before the 540 calibration. 541 542 This problem is particularly pronounced with an internal time source in Linux, 543 the kernel time, which is expressed in the theoretically high resolution 544 timespec - but which advances in much larger granularity intervals, sometimes 545 at the rate of jiffies, and possibly in catchup modes, at a much larger step. 546 547 This aliasing requires care in the computation and recalibration of kvmclock 548 and any other values derived from TSC computation (such as TSC virtualization 549 itself). 550 551 4.4) Migration 552 553 Migration of a virtual machine raises problems for timekeeping in two ways. 554 First, the migration itself may take time, during which interrupts cannot be 555 delivered, and after which, the guest time may need to be caught up. NTP may 556 be able to help to some degree here, as the clock correction required is 557 typically small enough to fall in the NTP-correctable window. 558 559 An additional concern is that timers based off the TSC (or HPET, if the raw bus 560 clock is exposed) may now be running at different rates, requiring compensation 561 in some way in the hypervisor by virtualizing these timers. In addition, 562 migrating to a faster machine may preclude the use of a passthrough TSC, as a 563 faster clock cannot be made visible to a guest without the potential of time 564 advancing faster than usual. A slower clock is less of a problem, as it can 565 always be caught up to the original rate. KVM clock avoids these problems by 566 simply storing multipliers and offsets against the TSC for the guest to convert 567 back into nanosecond resolution values. 568 569 4.5) Scheduling 570 571 Since scheduling may be based on precise timing and firing of interrupts, the 572 scheduling algorithms of an operating system may be adversely affected by 573 virtualization. In theory, the effect is random and should be universally 574 distributed, but in contrived as well as real scenarios (guest device access, 575 causes of virtualization exits, possible context switch), this may not always 576 be the case. The effect of this has not been well studied. 577 578 In an attempt to work around this, several implementations have provided a 579 paravirtualized scheduler clock, which reveals the true amount of CPU time for 580 which a virtual machine has been running. 581 582 4.6) Watchdogs 583 584 Watchdog timers, such as the lock detector in Linux may fire accidentally when 585 running under hardware virtualization due to timer interrupts being delayed or 586 misinterpretation of the passage of real time. Usually, these warnings are 587 spurious and can be ignored, but in some circumstances it may be necessary to 588 disable such detection. 589 590 4.7) Delays and precision timing 591 592 Precise timing and delays may not be possible in a virtualized system. This 593 can happen if the system is controlling physical hardware, or issues delays to 594 compensate for slower I/O to and from devices. The first issue is not solvable 595 in general for a virtualized system; hardware control software can't be 596 adequately virtualized without a full real-time operating system, which would 597 require an RT aware virtualization platform. 598 599 The second issue may cause performance problems, but this is unlikely to be a 600 significant issue. In many cases these delays may be eliminated through 601 configuration or paravirtualization. 602 603 4.8) Covert channels and leaks 604 605 In addition to the above problems, time information will inevitably leak to the 606 guest about the host in anything but a perfect implementation of virtualized 607 time. This may allow the guest to infer the presence of a hypervisor (as in a 608 red-pill type detection), and it may allow information to leak between guests 609 by using CPU utilization itself as a signalling channel. Preventing such 610 problems would require completely isolated virtual time which may not track 611 real time any longer. This may be useful in certain security or QA contexts, 612 but in general isn't recommended for real-world deployment scenarios.