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In fact, the QEMU code that uses KVM_GET_FPU/KVM_SET_FPU in x86 is
obsolete, because it's not been used since Linux 2.6.36.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

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