Based on kernel version 4.13.3. Page generated on 2017-09-23 13:56 EST.
1 The PPC KVM paravirtual interface 2 ================================= 3 4 The basic execution principle by which KVM on PowerPC works is to run all kernel 5 space code in PR=1 which is user space. This way we trap all privileged 6 instructions and can emulate them accordingly. 7 8 Unfortunately that is also the downfall. There are quite some privileged 9 instructions that needlessly return us to the hypervisor even though they 10 could be handled differently. 11 12 This is what the PPC PV interface helps with. It takes privileged instructions 13 and transforms them into unprivileged ones with some help from the hypervisor. 14 This cuts down virtualization costs by about 50% on some of my benchmarks. 15 16 The code for that interface can be found in arch/powerpc/kernel/kvm* 17 18 Querying for existence 19 ====================== 20 21 To find out if we're running on KVM or not, we leverage the device tree. When 22 Linux is running on KVM, a node /hypervisor exists. That node contains a 23 compatible property with the value "linux,kvm". 24 25 Once you determined you're running under a PV capable KVM, you can now use 26 hypercalls as described below. 27 28 KVM hypercalls 29 ============== 30 31 Inside the device tree's /hypervisor node there's a property called 32 'hypercall-instructions'. This property contains at most 4 opcodes that make 33 up the hypercall. To call a hypercall, just call these instructions. 34 35 The parameters are as follows: 36 37 Register IN OUT 38 39 r0 - volatile 40 r3 1st parameter Return code 41 r4 2nd parameter 1st output value 42 r5 3rd parameter 2nd output value 43 r6 4th parameter 3rd output value 44 r7 5th parameter 4th output value 45 r8 6th parameter 5th output value 46 r9 7th parameter 6th output value 47 r10 8th parameter 7th output value 48 r11 hypercall number 8th output value 49 r12 - volatile 50 51 Hypercall definitions are shared in generic code, so the same hypercall numbers 52 apply for x86 and powerpc alike with the exception that each KVM hypercall 53 also needs to be ORed with the KVM vendor code which is (42 << 16). 54 55 Return codes can be as follows: 56 57 Code Meaning 58 59 0 Success 60 12 Hypercall not implemented 61 <0 Error 62 63 The magic page 64 ============== 65 66 To enable communication between the hypervisor and guest there is a new shared 67 page that contains parts of supervisor visible register state. The guest can 68 map this shared page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE. 69 70 With this hypercall issued the guest always gets the magic page mapped at the 71 desired location. The first parameter indicates the effective address when the 72 MMU is enabled. The second parameter indicates the address in real mode, if 73 applicable to the target. For now, we always map the page to -4096. This way we 74 can access it using absolute load and store functions. The following 75 instruction reads the first field of the magic page: 76 77 ld rX, -4096(0) 78 79 The interface is designed to be extensible should there be need later to add 80 additional registers to the magic page. If you add fields to the magic page, 81 also define a new hypercall feature to indicate that the host can give you more 82 registers. Only if the host supports the additional features, make use of them. 83 84 The magic page layout is described by struct kvm_vcpu_arch_shared 85 in arch/powerpc/include/asm/kvm_para.h. 86 87 Magic page features 88 =================== 89 90 When mapping the magic page using the KVM hypercall KVM_HC_PPC_MAP_MAGIC_PAGE, 91 a second return value is passed to the guest. This second return value contains 92 a bitmap of available features inside the magic page. 93 94 The following enhancements to the magic page are currently available: 95 96 KVM_MAGIC_FEAT_SR Maps SR registers r/w in the magic page 97 KVM_MAGIC_FEAT_MAS0_TO_SPRG7 Maps MASn, ESR, PIR and high SPRGs 98 99 For enhanced features in the magic page, please check for the existence of the 100 feature before using them! 101 102 Magic page flags 103 ================ 104 105 In addition to features that indicate whether a host is capable of a particular 106 feature we also have a channel for a guest to tell the guest whether it's capable 107 of something. This is what we call "flags". 108 109 Flags are passed to the host in the low 12 bits of the Effective Address. 110 111 The following flags are currently available for a guest to expose: 112 113 MAGIC_PAGE_FLAG_NOT_MAPPED_NX Guest handles NX bits correctly wrt magic page 114 115 MSR bits 116 ======== 117 118 The MSR contains bits that require hypervisor intervention and bits that do 119 not require direct hypervisor intervention because they only get interpreted 120 when entering the guest or don't have any impact on the hypervisor's behavior. 121 122 The following bits are safe to be set inside the guest: 123 124 MSR_EE 125 MSR_RI 126 127 If any other bit changes in the MSR, please still use mtmsr(d). 128 129 Patched instructions 130 ==================== 131 132 The "ld" and "std" instructions are transformed to "lwz" and "stw" instructions 133 respectively on 32 bit systems with an added offset of 4 to accommodate for big 134 endianness. 135 136 The following is a list of mapping the Linux kernel performs when running as 137 guest. Implementing any of those mappings is optional, as the instruction traps 138 also act on the shared page. So calling privileged instructions still works as 139 before. 140 141 From To 142 ==== == 143 144 mfmsr rX ld rX, magic_page->msr 145 mfsprg rX, 0 ld rX, magic_page->sprg0 146 mfsprg rX, 1 ld rX, magic_page->sprg1 147 mfsprg rX, 2 ld rX, magic_page->sprg2 148 mfsprg rX, 3 ld rX, magic_page->sprg3 149 mfsrr0 rX ld rX, magic_page->srr0 150 mfsrr1 rX ld rX, magic_page->srr1 151 mfdar rX ld rX, magic_page->dar 152 mfdsisr rX lwz rX, magic_page->dsisr 153 154 mtmsr rX std rX, magic_page->msr 155 mtsprg 0, rX std rX, magic_page->sprg0 156 mtsprg 1, rX std rX, magic_page->sprg1 157 mtsprg 2, rX std rX, magic_page->sprg2 158 mtsprg 3, rX std rX, magic_page->sprg3 159 mtsrr0 rX std rX, magic_page->srr0 160 mtsrr1 rX std rX, magic_page->srr1 161 mtdar rX std rX, magic_page->dar 162 mtdsisr rX stw rX, magic_page->dsisr 163 164 tlbsync nop 165 166 mtmsrd rX, 0 b <special mtmsr section> 167 mtmsr rX b <special mtmsr section> 168 169 mtmsrd rX, 1 b <special mtmsrd section> 170 171 [Book3S only] 172 mtsrin rX, rY b <special mtsrin section> 173 174 [BookE only] 175 wrteei [0|1] b <special wrteei section> 176 177 178 Some instructions require more logic to determine what's going on than a load 179 or store instruction can deliver. To enable patching of those, we keep some 180 RAM around where we can live translate instructions to. What happens is the 181 following: 182 183 1) copy emulation code to memory 184 2) patch that code to fit the emulated instruction 185 3) patch that code to return to the original pc + 4 186 4) patch the original instruction to branch to the new code 187 188 That way we can inject an arbitrary amount of code as replacement for a single 189 instruction. This allows us to check for pending interrupts when setting EE=1 190 for example. 191 192 Hypercall ABIs in KVM on PowerPC 193 ================================= 194 1) KVM hypercalls (ePAPR) 195 196 These are ePAPR compliant hypercall implementation (mentioned above). Even 197 generic hypercalls are implemented here, like the ePAPR idle hcall. These are 198 available on all targets. 199 200 2) PAPR hypercalls 201 202 PAPR hypercalls are needed to run server PowerPC PAPR guests (-M pseries in QEMU). 203 These are the same hypercalls that pHyp, the POWER hypervisor implements. Some of 204 them are handled in the kernel, some are handled in user space. This is only 205 available on book3s_64. 206 207 3) OSI hypercalls 208 209 Mac-on-Linux is another user of KVM on PowerPC, which has its own hypercall (long 210 before KVM). This is supported to maintain compatibility. All these hypercalls get 211 forwarded to user space. This is only useful on book3s_32, but can be used with 212 book3s_64 as well.