linux-kernel - Re: [PATCH v4 3/8] x86/bugs: Use an X86_FEATURE_xxx flag for the MMIO Stale Data mitigation

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Message-ID: <aQU6LqP-PxBQ-R0m@google.com>
Date: Fri, 31 Oct 2025 15:37:34 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Josh Poimboeuf <jpoimboe@...nel.org>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Brendan Jackman <jackmanb@...gle.com>
Subject: Re: [PATCH v4 3/8] x86/bugs: Use an X86_FEATURE_xxx flag for the MMIO
 Stale Data mitigation

On Fri, Oct 31, 2025, Pawan Gupta wrote:
> On Thu, Oct 30, 2025 at 05:30:35PM -0700, Sean Christopherson wrote:
> > Convert the MMIO Stale Data mitigation flag from a static branch into an
> > X86_FEATURE_xxx so that it can be used via ALTERNATIVE_2 in KVM.
> > 
> > No functional change intended.
> > 
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > ---
> >  arch/x86/include/asm/cpufeatures.h   |  1 +
> >  arch/x86/include/asm/nospec-branch.h |  2 --
> >  arch/x86/kernel/cpu/bugs.c           | 11 +----------
> >  arch/x86/kvm/mmu/spte.c              |  2 +-
> >  arch/x86/kvm/vmx/vmx.c               |  4 ++--
> >  5 files changed, 5 insertions(+), 15 deletions(-)
> > 
> > diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
> > index 7129eb44adad..d1d7b5ec6425 100644
> > --- a/arch/x86/include/asm/cpufeatures.h
> > +++ b/arch/x86/include/asm/cpufeatures.h
> > @@ -501,6 +501,7 @@
> >  #define X86_FEATURE_ABMC		(21*32+15) /* Assignable Bandwidth Monitoring Counters */
> >  #define X86_FEATURE_MSR_IMM		(21*32+16) /* MSR immediate form instructions */
> >  #define X86_FEATURE_X2AVIC_EXT		(21*32+17) /* AMD SVM x2AVIC support for 4k vCPUs */
> > +#define X86_FEATURE_CLEAR_CPU_BUF_MMIO	(21*32+18) /* Clear CPU buffers using VERW before VMRUN, iff the vCPU can access host MMIO*/
> 
> Some bikeshedding from my side too:
> s/iff/if/

Heh, that's actually intentional.  "iff" is shorthand for "if and only if".  But
this isn't the first time my use of "iff" has confused people, so I've no objection
to switching to "if".