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: <DDZ01Y0TC9J2.26YWWKHOKKGSG@google.com>
Date: Mon, 03 Nov 2025 10:49:20 +0000
From: Brendan Jackman <jackmanb@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, 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 at 9:47 PM UTC, Sean Christopherson wrote:
> 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.

Ah, after finishing the review I realised these are _not_ actually
mutually exclusive in terms of the implementation. So asserting here
that they are mutually exclusive would just be confusing, rather than
helfpul, IMO.