linux-kernel - Re: [PATCH 2/5] x86, fpu: separate fpstate->xfd and guest XFD

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Message-ID: <aVMS1xa99GsiZpFQ@google.com>
Date: Mon, 29 Dec 2025 15:46:31 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, x86@...nel.org, 
	stable@...r.kernel.org
Subject: Re: [PATCH 2/5] x86, fpu: separate fpstate->xfd and guest XFD

On Tue, Dec 30, 2025, Paolo Bonzini wrote:
> On Mon, Dec 29, 2025 at 11:45 PM Sean Christopherson <seanjc@...gle.com> wrote:
> > So, given that KVM's effective ABI is to record XSTATE_BV[i]=0 if XFD[i]==1, I
> > vote to fix this by emulating that behavior when stuffing XFD in
> > fpu_update_guest_xfd(), and then manually closing the hole Paolo found in
> > fpu_copy_uabi_to_guest_fpstate().
> 
> I disagree with changing the argument from const void* to void*.
> Let's instead treat it as a KVM backwards-compatibility quirk:
> 
>     union fpregs_state *xstate =
>         (union fpregs_state *)guest_xsave->region;
>     xstate->xsave.header.xfeatures &=
>         ~vcpu->arch.guest_fpu.fpstate->xfd;
> 
> It keeps the kernel/ API const as expected and if anything I'd
> consider adding a WARN to fpu_copy_uabi_to_guest_fpstate(), basically
> asserting that there would be no #NM on the subsequent restore.

Works for me.  

> > @@ -319,10 +319,25 @@ EXPORT_SYMBOL_FOR_KVM(fpu_enable_guest_xfd_features);
> >  #ifdef CONFIG_X86_64
> >  void fpu_update_guest_xfd(struct fpu_guest *guest_fpu, u64 xfd)
> >  {
> > +       struct fpstate *fpstate = guest_fpu->fpstate;
> > +
> >         fpregs_lock();
> > -       guest_fpu->fpstate->xfd = xfd;
> > -       if (guest_fpu->fpstate->in_use)
> > -               xfd_update_state(guest_fpu->fpstate);
> > +       fpstate->xfd = xfd;
> > +       if (fpstate->in_use)
> > +               xfd_update_state(fpstate);
> > +
> > +       /*
> > +        * If the guest's FPU state is NOT resident in hardware, clear disabled
> > +        * components in XSTATE_BV as attempting to load disabled components
> > +        * will generate #NM _in the host_, and KVM's ABI is that saving guest
> > +        * XSAVE state should see XSTATE_BV[i]=0 if XFD[i]=1.
> > +        *
> > +        * If the guest's FPU state is in hardware, simply do nothing as XSAVE
> > +        * itself saves XSTATE_BV[i] as 0 if XFD[i]=1.
> 
> s/saves/(from fpu_swap_kvm_fpstate) will save/
> 
> > +        */
> > +       if (xfd && test_thread_flag(TIF_NEED_FPU_LOAD))
> > +               fpstate->regs.xsave.header.xfeatures &= ~xfd;
> 
> No objections to this part.  I'll play with this to adjust the
> selftests either tomorrow or, more likely, on January 2nd, and send a
> v2 that also includes the change from preemption_disabled to
> irqs_disabled.

To hopefully save you some time, I responded to the selftests with cleanups and
adjustments to hit both bugs (see patch 3).

> I take it that you don't have any qualms with the new
> fpu_load_guest_fpstate function,

Hmm, I don't have a strong opinion?  Actually, after looking at patch 5, I agree
that adding fpu_load_guest_fpstate() is useful.  My only hesitation was that
kvm_fpu_{get,put}() would be _very_ similar, but critically different, at which
point NOT using fpu_update_guest_xfd() in kvm_fpu_get() could be confusing.

> but let me know if you prefer to have it in a separate submission destined to
> 6.20 only.

I'd say don't send it to stable@, otherwise I don't have a preference on 6.19
versus 6.20.