linux-kernel - Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and vmx_l1d_flush

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Message-ID: <aO6fOM-XydknM6NJ@google.com>
Date: Tue, 14 Oct 2025 12:06:32 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Valentin Schneider <vschneid@...hat.com>
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Subject: Re: [PATCH v6 19/29] KVM: VMX: Mark vmx_l1d_should flush and
 vmx_l1d_flush_cond keys as allowed in .noinstr

On Tue, Oct 14, 2025, Valentin Schneider wrote:
> On 13/10/25 17:01, Sean Christopherson wrote:
> > On Fri, Oct 10, 2025, Valentin Schneider wrote:
> >> Later commits will cause objtool to warn about static keys being used in
> >> .noinstr sections in order to safely defer instruction patching IPIs
> >> targeted at NOHZ_FULL CPUs.
> >>
> >> These keys are used in .noinstr code, and can be modified at runtime
> >> (/proc/kernel/vmx* write). However it is not expected that they will be
> >> flipped during latency-sensitive operations, and thus shouldn't be a source
> >> of interference wrt the text patching IPI.
> >>
> >> Mark it to let objtool know not to warn about it.
> >
> > Can you elaborate in the changelog on what will happen if the key is toggle?
> > IIUC, smp_text_poke_batch_finish() will force IPIs if noinstr code is being
> > patched.
> 
> Right!
> 
> > Even just a small footnote like this:
> >
> >   Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
> >   being patched, i.e. this is purely about silencing objtool warnings.
> >
> > to make it clear that there's no bug/race being introduced.
> 
> Good point. How about:
> 
> """
> Later commits will cause objtool to warn about static keys being used in
> .noinstr sections in order to safely defer instruction patching IPIs
> targeted at NOHZ_FULL CPUs.
> 
> The VMX keys are used in .noinstr code, and can be modified at runtime
> (/proc/kernel/vmx* write). However it is not expected that they will be
> flipped during latency-sensitive operations, and thus shouldn't be a source
> of interference for NOHZ_FULL CPUs wrt the text patching IPI.
> 
> Note, smp_text_poke_batch_finish() never defers IPIs if noinstr code is
> being patched, i.e. this is purely to tell objtool we're okay with updates
> to that key causing IPIs and to silence the associated objtool warning.
> """

LGTM.  With the updated changelog,

Acked-by: Sean Christopherson <seanjc@...gle.com>