linux-kernel - Re: [PATCH] KVM: x86: Always enable legacy fp/sse

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Date:   Wed, 17 Aug 2022 10:45:14 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Leonardo Brás <leobras@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        "Dr. David Alan Gilbert (git)" <dgilbert@...hat.com>
Cc:     kvm@...r.kernel.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org
Subject: Re: [PATCH] KVM: x86: Always enable legacy fp/sse

On 8/17/22 05:29, Leonardo Brás wrote:
>>> QEMU always calls kvm_put_xsave, even on this combination because
>>> KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
> Any particular reason why it always returns true for KVM_CAP_XSAVE, even when
> the CPU does not support it?
> 
> IIUC, if it returns false to this capability, kvm_put_xsave() should never be
> called, and thus it can avoid bug reproduction.

Because it allows userspace to have a single path for saving/restoring 
FPU state.  See for example the "migration" code in 
tools/testing/selftests/kvm/lib/x86_64/processor.c (the vcpu_save_state 
and vcpu_load_state functions).

In fact, the QEMU code that uses KVM_GET_FPU/KVM_SET_FPU in x86 is 
obsolete, because it's not been used since Linux 2.6.36.

Paolo