Re: [PATCH] KVM: x86: workaround invalid CPUID[0xD,9] info on some AMD processors

[Daniel P . Berrangé]

On Wed, Mar 23, 2022 at 12:43:15PM +0100, Paolo Bonzini wrote:
> Some AMD processors expose the PKRU extended save state even if they do not 
> have
> the related PKU feature in CPUID.  Worse, when they do they report a size of
> 64, whereas the expected size of the PKRU extended save state is 8, therefore
> the esa->size == eax assertion does not hold.
> 
> The state is already ignored by KVM_GET_SUPPORTED_CPUID because it
> was not enabled in the host XCR0.  However, QEMU kvm_cpu_xsave_init()
> runs before QEMU invokes arch_prctl() to enable dynamically-enabled
> save states such as XTILEDATA, and KVM_GET_SUPPORTED_CPUID hides save
> states that have yet to be enabled.  Therefore, kvm_cpu_xsave_init()
> needs to consult the host CPUID instead of KVM_GET_SUPPORTED_CPUID,
> and dies with an assertion failure.
> 
> When setting up the ExtSaveArea array to match the host, ignore features that
> KVM does not report as supported.  This will cause QEMU to skip the incorrect
> CPUID leaf instead of tripping the assertion.

  Closes: https://gitlab.com/qemu-project/qemu/-/issues/916

> 
> Reported-by: Daniel P. Berrangé <berrange@redhat.com>

Also credit

  Reported-by: Peter Krempa <pkrempa@redhat.com>

> Analyzed-by: Yang Zhong <yang.zhong@intel.com>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  target/i386/cpu.c         |  4 ++--
>  target/i386/cpu.h         |  2 ++
>  target/i386/kvm/kvm-cpu.c | 19 ++++++++++++-------
>  3 files changed, 16 insertions(+), 9 deletions(-)

  Tested-by: Daniel P. Berrangé <berrange@redhat.com>

no longer crashes on the AMD machine I have to hand.


With regards,
Daniel
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