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: NPaolo Bonzini <pbonzini@redhat.com>
Reviewed-by: NJim Mattson <jmattson@google.com>
Reviewed-by: NOliver Upton <oupton@google.com>
Message-Id: <20220322110712.222449-3-pbonzini@redhat.com>
Signed-off-by: NPaolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/x86/errata.rst

+
+=======================================
+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
+

Documentation/virt/kvm/x86/index.rst

@@ -9,6 +9,7 @@ KVM for x86 systems
 
    amd-memory-encryption
    cpuid
+   errata
    halt-polling
    hypercalls
    mmu