diff options
| author | Paolo Bonzini <pbonzini@redhat.com> | 2022-03-22 12:07:11 +0100 |
|---|---|---|
| committer | Paolo Bonzini <pbonzini@redhat.com> | 2022-03-29 13:21:20 -0400 |
| commit | 45016721de3c714902c6f475b705e10ae0bdd801 (patch) | |
| tree | 1464ac76fed7b5e26548c39d9a3b39d443d541a5 | |
| parent | daec8d408308ee7322d86cdd2dc3332e9cdbedf9 (diff) | |
Documentation: KVM: add virtual CPU errata documentation
Add a file to document all the different ways in which the virtual CPU
emulation is imperfect. Include an example to show how to document
such errata.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20220322110712.222449-3-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
| -rw-r--r-- | Documentation/virt/kvm/x86/errata.rst | 39 | ||||
| -rw-r--r-- | Documentation/virt/kvm/x86/index.rst | 1 |
2 files changed, 40 insertions, 0 deletions
diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst new file mode 100644 index 000000000000..806f049b6975 --- /dev/null +++ b/Documentation/virt/kvm/x86/errata.rst @@ -0,0 +1,39 @@ + +======================================= +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 + diff --git a/Documentation/virt/kvm/x86/index.rst b/Documentation/virt/kvm/x86/index.rst index 55ede8e070b6..7ff588826b9f 100644 --- a/Documentation/virt/kvm/x86/index.rst +++ b/Documentation/virt/kvm/x86/index.rst @@ -9,6 +9,7 @@ KVM for x86 systems amd-memory-encryption cpuid + errata halt-polling hypercalls mmu |