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 <[email protected]>
---
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
On Tue, Mar 22, 2022 at 4:07 AM Paolo Bonzini <[email protected]> wrote:
>
> 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.
This is fantastic. Thank you. Along these lines, I wonder if we should
come up with a mechanism a la IA32_ARCH_CAPABILITIES (or AMD's OSVW
MSRs) for declaring fixed errata.
For example, today, KVM ignores the guest-only and host-only bits in
PerfEvtSel MSRs, but I can't imagine we're going to do that forever.
Reviewed-by: Jim Mattson <[email protected]>
Hi Paolo,
On Tue, Mar 22, 2022 at 12:07:11PM +0100, Paolo Bonzini wrote:
> 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 <[email protected]>
+100, this is excellent!
Reviewed-by: Oliver Upton <[email protected]>
> ---
> 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
> +=======================================
In an attempt for us KVM developers to save face, would it make
sense to rip the table for KVM_CAP_DISABLE_QUIRKS2 out of api.rst and
drop it here?
That way, we can discuss all of the known errata in KVM *and* mention
the chicken bits we provide to mitigate :)
--
Thanks,
Oliver