linux-kernel - Re: [PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever XFD[i]=1

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Message-ID: <CABgObfbtdSnzRCsiDHgjnT5OTROMOEgWZL+AMOSFj2+hXOsATw@mail.gmail.com>
Date: Thu, 8 Jan 2026 17:26:32 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, seanjc@...gle.com, 
	x86@...nel.org, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever XFD[i]=1

On Thu, Jan 8, 2026 at 4:08 AM Binbin Wu <binbin.wu@...ux.intel.com> wrote:
> > +     /*
> > +      * KVM's guest ABI is that setting XFD[i]=1 *can* immediately revert
> > +      * the save state to initialized.  Likewise, KVM_GET_XSAVE does the
>
> Nit:
> To me "initialized" has the implication that it's active.
> I prefer the description "initial state" or "initial configuration" used in
> SDM here.
> I am not a native English speaker though, please ignore it if it's just my
> feeling.

Sure, why not:

   KVM's guest ABI is that setting XFD[i]=1 *can* immediately revert the
   save state to its initial configuration.  Likewise, KVM_GET_XSAVE does
   the same as XSAVE and returns XSTATE_BV[i]=0 whenever XFD[i]=1.

> > +     /*
> > +      * Do not reject non-initialized disabled features for backwards
> > +      * compatibility, but clear XSTATE_BV[i] whenever XFD[i]=1.
> > +      * Otherwise, XRSTOR would cause a #NM.
> > +      */

Same here:

   For backwards compatibility, do not expect disabled features to be in
   their initial state.  XSTATE_BV[i] must still be cleared whenever
   XFD[i]=1, or XRSTOR would cause a #NM.

Thanks!

Paolo