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: <20251112144655.GZaRSd32GC0lZdDOWg@fat_crate.local>
Date: Wed, 12 Nov 2025 15:46:55 +0100
From: Borislav Petkov <bp@...en8.de>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	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 Thu, Oct 30, 2025 at 05:30:35PM -0700, Sean Christopherson wrote:
> Subject: Re: [PATCH v4 3/8] x86/bugs: Use an X86_FEATURE_xxx flag for the MMIO Stale Data mitigation

I'm guessing that "xxx" would turn into the proper name after we're done
bikeshedding.

> 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*/
							   ^^^^^^^

Yes, you can break the line and format it properly. :-)

Also, this should be called then

X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO

as it is a VM-thing too.

Also, in my tree pile I have for bit 17

#define X86_FEATURE_SGX_EUPDATESVN      (21*32+17) /* Support for ENCLS[EUPDATESVN] instruction */

I see you have X86_FEATURE_X2AVIC_EXT there so we need to pay attention during
the merge window.

-- 
Regards/Gruss,
    Boris.

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