| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======================================= |
| Known limitations of CPU virtualization |
| ======================================= |
| |
| Whenever perfect emulation of a CPU feature is impossible or too hard, KVM |
| has to choose between not implementing the feature at all or introducing |
| behavioral differences between virtual machines and bare metal systems. |
| |
| This file documents some of the known limitations that KVM has in |
| virtualizing CPU features. |
| |
| x86 |
| === |
| |
| ``KVM_GET_SUPPORTED_CPUID`` issues |
| ---------------------------------- |
| |
| x87 features |
| ~~~~~~~~~~~~ |
| |
| Unlike most other CPUID feature bits, CPUID[EAX=7,ECX=0]:EBX[6] |
| (FDP_EXCPTN_ONLY) and CPUID[EAX=7,ECX=0]:EBX]13] (ZERO_FCS_FDS) are |
| clear if the features are present and set if the features are not present. |
| |
| Clearing these bits in CPUID has no effect on the operation of the guest; |
| if these bits are set on hardware, the features will not be present on |
| any virtual machine that runs on that hardware. |
| |
| **Workaround:** It is recommended to always set these bits in guest CPUID. |
| Note however that any software (e.g ``WIN87EM.DLL``) expecting these features |
| to be present likely predates these CPUID feature bits, and therefore |
| doesn't know to check for them anyway. |
| |
| Nested virtualization features |
| ------------------------------ |
| |
| TBD |
| |
| x2APIC |
| ------ |
| When KVM_X2APIC_API_USE_32BIT_IDS is enabled, KVM activates a hack/quirk that |
| allows sending events to a single vCPU using its x2APIC ID even if the target |
| vCPU has legacy xAPIC enabled, e.g. to bring up hotplugged vCPUs via INIT-SIPI |
| on VMs with > 255 vCPUs. A side effect of the quirk is that, if multiple vCPUs |
| have the same physical APIC ID, KVM will deliver events targeting that APIC ID |
| only to the vCPU with the lowest vCPU ID. If KVM_X2APIC_API_USE_32BIT_IDS is |
| not enabled, KVM follows x86 architecture when processing interrupts (all vCPUs |
| matching the target APIC ID receive the interrupt). |
| |
| MTRRs |
| ----- |
| KVM does not virtualize guest MTRR memory types. KVM emulates accesses to MTRR |
| MSRs, i.e. {RD,WR}MSR in the guest will behave as expected, but KVM does not |
| honor guest MTRRs when determining the effective memory type, and instead |
| treats all of guest memory as having Writeback (WB) MTRRs. |
| |
| CR0.CD |
| ------ |
| KVM does not virtualize CR0.CD on Intel CPUs. Similar to MTRR MSRs, KVM |
| emulates CR0.CD accesses so that loads and stores from/to CR0 behave as |
| expected, but setting CR0.CD=1 has no impact on the cachaeability of guest |
| memory. |
| |
| Note, this erratum does not affect AMD CPUs, which fully virtualize CR0.CD in |
| hardware, i.e. put the CPU caches into "no fill" mode when CR0.CD=1, even when |
| running in the guest. |
