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

[Leonardo Brás]

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@xxxxxxxxxx>
> > 
> > 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@xxxxxxxxxx>
> > ---
> >  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
> --
>