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: <aQUuYqowQtbRCYyC@google.com>
Date: Fri, 31 Oct 2025 14:47:14 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Brendan Jackman <jackmanb@...gle.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, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.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, Brendan Jackman wrote:
> On Fri Oct 31, 2025 at 12:30 AM UTC, Sean Christopherson wrote:
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 723666a1357e..9acf6343b0ac 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -192,14 +192,6 @@ EXPORT_SYMBOL_GPL(cpu_buf_idle_clear);
> >   */
> >  DEFINE_STATIC_KEY_FALSE(switch_mm_cond_l1d_flush);
> >  
> > -/*
> > - * Controls CPU Fill buffer clear before VMenter. This is a subset of
> > - * X86_FEATURE_CLEAR_CPU_BUF_VM, and should only be enabled when KVM-only
> > - * mitigation is required.
> > - */
> 
> This comment wasn't super clear IMO but now that we're losing it, maybe
> we can replace it with a WARN_ON() at the end of
> cpu_apply_mitigations() or something (maybe it belongs in VMX code)? To
> make it more obvious that X86_FEATURE_CLEAR_CPU_BUF_VM and
> X86_FEATURE_CLEAR_CPU_BUF_MMIO are mutually exclusive.

No objection from me if we want strong guarantees that CLEAR_CPU_BUF_VM and
CLEAR_CPU_BUF_MMIO are mutually exclusive.  Though I do think the KVM side of
things (and the kernel in general) should be paranoid and not lean _too_ hard
on such assumptions.