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SubjectRe: [PATCH] KVM: x86: Always enable legacy fp/sse
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On Tue, 2022-08-16 at 21:37 +0000, Sean Christopherson wrote:
> On Tue, Aug 16, 2022, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> >
> > A live migration under qemu is currently failing when the source
> > host is ~Nehalem era (pre-xsave) and the destination is much newer,
> > (configured with a guest CPU type of Nehalem).
> > QEMU always calls kvm_put_xsave, even on this combination because
> > KVM_CAP_CHECK_EXTENSION_VM always returns true for KVM_CAP_XSAVE.
> >
> > When QEMU calls kvm_put_xsave it's rejected by
> > fpu_copy_uabi_to_guest_fpstate->
> > copy_uabi_to_xstate->
> > validate_user_xstate_header
> >
> > when the validate checks the loaded xfeatures against
> > user_xfeatures, which it finds to be 0.
> >
> > I think our initialisation of user_xfeatures is being
> > too strict here, and we should always allow the base FP/SSE.
> >
> > Fixes: ad856280ddea ("x86/kvm/fpu: Limit guest user_xfeatures to supported bits of XCR0")

Thanks for fixing this, Dave!

> > bz: https://bugzilla.redhat.com/show_bug.cgi?id=2079311
> >
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> > arch/x86/kvm/cpuid.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
> > index de6d44e07e34..3b2319cecfd1 100644
> > --- a/arch/x86/kvm/cpuid.c
> > +++ b/arch/x86/kvm/cpuid.c
> > @@ -298,7 +298,8 @@ static void kvm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> > guest_supported_xcr0 =
> > cpuid_get_supported_xcr0(vcpu->arch.cpuid_entries, vcpu->arch.cpuid_nent);
> >
> > - vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0;
> > + vcpu->arch.guest_fpu.fpstate->user_xfeatures = guest_supported_xcr0 |
> > + XFEATURE_MASK_FPSSE;
>
> I don't think this is correct. This will allow the guest to set the SSE bit
> even when XSAVE isn't supported due to kvm_guest_supported_xcr0() returning
> user_xfeatures.
>
> static inline u64 kvm_guest_supported_xcr0(struct kvm_vcpu *vcpu)
> {
> return vcpu->arch.guest_fpu.fpstate->user_xfeatures;
> }
>
> I believe the right place to fix this is in validate_user_xstate_header(). It's
> reachable if and only if XSAVE is supported in the host, and when XSAVE is _not_
> supported, the kernel unconditionally allows FP+SSE. So it follows that the kernel
> should also allow FP+SSE when using XSAVE too. That would also align the logic
> with fpu_copy_guest_fpstate_to_uabi(), which fordces the FPSSE flags. Ditto for
> the non-KVM save_xstate_epilog().
>
> Aha! And fpu__init_system_xstate() ensure the host supports FP+SSE when XSAVE
> is enabled (knew their had to be a sanity check somewhere).

Thanks for the feedback Sean!

I have near to no experience in this code, and I hope you can help me with a
question I have, based in Dave's commit message:

> > 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.

Thanks in advance,

Leo
>
> ---
> arch/x86/kernel/fpu/xstate.c | 9 +++++++--
> 1 file changed, 7 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kernel/fpu/xstate.c b/arch/x86/kernel/fpu/xstate.c
> index c8340156bfd2..83b9a9653d47 100644
> --- a/arch/x86/kernel/fpu/xstate.c
> +++ b/arch/x86/kernel/fpu/xstate.c
> @@ -399,8 +399,13 @@ int xfeature_size(int xfeature_nr)
> static int validate_user_xstate_header(const struct xstate_header *hdr,
> struct fpstate *fpstate)
> {
> - /* No unknown or supervisor features may be set */
> - if (hdr->xfeatures & ~fpstate->user_xfeatures)
> + /*
> + * No unknown or supervisor features may be set. Userspace is always
> + * allowed to restore FP+SSE state (XSAVE/XRSTOR are used by the kernel
> + * if and only if FP+SSE are supported in xstate).
> + */
> + if (hdr->xfeatures & ~fpstate->user_xfeatures &
> + ~(XFEATURE_MASK_FP | XFEATURE_MASK_SSE))
> return -EINVAL;
>
> /* Userspace must use the uncompacted format */
>
> base-commit: de3d415edca23831c5d1f24f10c74a715af7efdb
> --
>

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