Based on kernel version 3.19. Page generated on 2015-02-13 21:21 EST.
1 Paravirt_ops on IA64 2 ==================== 3 21 May 2008, Isaku Yamahata <firstname.lastname@example.org> 4 5 6 Introduction 7 ------------ 8 The aim of this documentation is to help with maintainability and/or to 9 encourage people to use paravirt_ops/IA64. 10 11 paravirt_ops (pv_ops in short) is a way for virtualization support of 12 Linux kernel on x86. Several ways for virtualization support were 13 proposed, paravirt_ops is the winner. 14 On the other hand, now there are also several IA64 virtualization 15 technologies like kvm/IA64, xen/IA64 and many other academic IA64 16 hypervisors so that it is good to add generic virtualization 17 infrastructure on Linux/IA64. 18 19 20 What is paravirt_ops? 21 --------------------- 22 It has been developed on x86 as virtualization support via API, not ABI. 23 It allows each hypervisor to override operations which are important for 24 hypervisors at API level. And it allows a single kernel binary to run on 25 all supported execution environments including native machine. 26 Essentially paravirt_ops is a set of function pointers which represent 27 operations corresponding to low level sensitive instructions and high 28 level functionalities in various area. But one significant difference 29 from usual function pointer table is that it allows optimization with 30 binary patch. It is because some of these operations are very 31 performance sensitive and indirect call overhead is not negligible. 32 With binary patch, indirect C function call can be transformed into 33 direct C function call or in-place execution to eliminate the overhead. 34 35 Thus, operations of paravirt_ops are classified into three categories. 36 - simple indirect call 37 These operations correspond to high level functionality so that the 38 overhead of indirect call isn't very important. 39 40 - indirect call which allows optimization with binary patch 41 Usually these operations correspond to low level instructions. They 42 are called frequently and performance critical. So the overhead is 43 very important. 44 45 - a set of macros for hand written assembly code 46 Hand written assembly codes (.S files) also need paravirtualization 47 because they include sensitive instructions or some of code paths in 48 them are very performance critical. 49 50 51 The relation to the IA64 machine vector 52 --------------------------------------- 53 Linux/IA64 has the IA64 machine vector functionality which allows the 54 kernel to switch implementations (e.g. initialization, ipi, dma api...) 55 depending on executing platform. 56 We can replace some implementations very easily defining a new machine 57 vector. Thus another approach for virtualization support would be 58 enhancing the machine vector functionality. 59 But paravirt_ops approach was taken because 60 - virtualization support needs wider support than machine vector does. 61 e.g. low level instruction paravirtualization. It must be 62 initialized very early before platform detection. 63 64 - virtualization support needs more functionality like binary patch. 65 Probably the calling overhead might not be very large compared to the 66 emulation overhead of virtualization. However in the native case, the 67 overhead should be eliminated completely. 68 A single kernel binary should run on each environment including native, 69 and the overhead of paravirt_ops on native environment should be as 70 small as possible. 71 72 - for full virtualization technology, e.g. KVM/IA64 or 73 Xen/IA64 HVM domain, the result would be 74 (the emulated platform machine vector. probably dig) + (pv_ops). 75 This means that the virtualization support layer should be under 76 the machine vector layer. 77 78 Possibly it might be better to move some function pointers from 79 paravirt_ops to machine vector. In fact, Xen domU case utilizes both 80 pv_ops and machine vector. 81 82 83 IA64 paravirt_ops 84 ----------------- 85 In this section, the concrete paravirt_ops will be discussed. 86 Because of the architecture difference between ia64 and x86, the 87 resulting set of functions is very different from x86 pv_ops. 88 89 - C function pointer tables 90 They are not very performance critical so that simple C indirect 91 function call is acceptable. The following structures are defined at 92 this moment. For details see linux/include/asm-ia64/paravirt.h 93 - struct pv_info 94 This structure describes the execution environment. 95 - struct pv_init_ops 96 This structure describes the various initialization hooks. 97 - struct pv_iosapic_ops 98 This structure describes hooks to iosapic operations. 99 - struct pv_irq_ops 100 This structure describes hooks to irq related operations 101 - struct pv_time_op 102 This structure describes hooks to steal time accounting. 103 104 - a set of indirect calls which need optimization 105 Currently this class of functions correspond to a subset of IA64 106 intrinsics. At this moment the optimization with binary patch isn't 107 implemented yet. 108 struct pv_cpu_op is defined. For details see 109 linux/include/asm-ia64/paravirt_privop.h 110 Mostly they correspond to ia64 intrinsics 1-to-1. 111 Caveat: Now they are defined as C indirect function pointers, but in 112 order to support binary patch optimization, they will be changed 113 using GCC extended inline assembly code. 114 115 - a set of macros for hand written assembly code (.S files) 116 For maintenance purpose, the taken approach for .S files is single 117 source code and compile multiple times with different macros definitions. 118 Each pv_ops instance must define those macros to compile. 119 The important thing here is that sensitive, but non-privileged 120 instructions must be paravirtualized and that some privileged 121 instructions also need paravirtualization for reasonable performance. 122 Developers who modify .S files must be aware of that. At this moment 123 an easy checker is implemented to detect paravirtualization breakage. 124 But it doesn't cover all the cases. 125 126 Sometimes this set of macros is called pv_cpu_asm_op. But there is no 127 corresponding structure in the source code. 128 Those macros mostly 1:1 correspond to a subset of privileged 129 instructions. See linux/include/asm-ia64/native/inst.h. 130 And some functions written in assembly also need to be overrided so 131 that each pv_ops instance have to define some macros. Again see 132 linux/include/asm-ia64/native/inst.h. 133 134 135 Those structures must be initialized very early before start_kernel. 136 Probably initialized in head.S using multi entry point or some other trick. 137 For native case implementation see linux/arch/ia64/kernel/paravirt.c.