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

[Pawan Gupta]

On Fri, Oct 31, 2025 at 03:37:34PM -0700, Sean Christopherson wrote:
> 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@xxxxxxxxxx>
> > > ---
> > >  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".

I did a quick search, there are about ~500 instances of "iff" in the
kernel. So, it's a common abbreviation that I learnt today. It is fine to
keep it as is.