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Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
To:     linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Cc:     mlevitsk@redhat.com, jmattson@google.com
Subject: [PATCH 2/3] Documentation: KVM: add virtual CPU errata documentation
Date:   Tue, 22 Mar 2022 12:07:11 +0100
Message-Id: <20220322110712.222449-3-pbonzini@redhat.com>
In-Reply-To: <20220322110712.222449-1-pbonzini@redhat.com>
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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>
---
 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

diff --git a/Documentation/virt/kvm/x86/errata.rst b/Documentation/virt/kvm/x86/errata.rst
new file mode 100644
index 000000000000..df394d34a836
--- /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
-- 
2.35.1