From 45016721de3c714902c6f475b705e10ae0bdd801 Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Tue, 22 Mar 2022 12:07:11 +0100 Subject: 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 Reviewed-by: Jim Mattson Reviewed-by: Oliver Upton Message-Id: <20220322110712.222449-3-pbonzini@redhat.com> Signed-off-by: Paolo Bonzini --- Documentation/virt/kvm/x86/errata.rst | 39 +++++++++++++++++++++++++++++++++++ Documentation/virt/kvm/x86/index.rst | 1 + 2 files changed, 40 insertions(+) create mode 100644 Documentation/virt/kvm/x86/errata.rst (limited to 'Documentation/virt') 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 -- cgit v1.2.3