RE: [RFC PATCH 11/56] x86/bugs: Reset MMIO mitigations

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Borislav Petkov <bp@xxxxxxxxx>
> Sent: Monday, January 26, 2026 7:06 AM
> To: Kaplan, David <David.Kaplan@xxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Peter Zijlstra
> <peterz@xxxxxxxxxxxxx>; Josh Poimboeuf <jpoimboe@xxxxxxxxxx>; Pawan
> Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>; Ingo Molnar
> <mingo@xxxxxxxxxx>; Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>;
> x86@xxxxxxxxxx; H . Peter Anvin <hpa@xxxxxxxxx>; Alexander Graf
> <graf@xxxxxxxxxx>; Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [RFC PATCH 11/56] x86/bugs: Reset MMIO mitigations
>
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>
> On Mon, Oct 13, 2025 at 09:33:59AM -0500, David Kaplan wrote:
> > Add function to reset MMIO mitigations back to their boot-time defaults.
> >
> > Signed-off-by: David Kaplan <david.kaplan@xxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 11 +++++++++++
> >  1 file changed, 11 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 5668a8b8821b..9139c8187913 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -787,6 +787,16 @@ static void __init mmio_apply_mitigation(void)
> >               cpu_smt_disable(false);
> >  }
> >
> > +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> > +static void mmio_reset_mitigation(void)
> > +{
> > +     static_branch_disable(&cpu_buf_vm_clear);
>
> This needs to be
>
>         setup_clear_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF_VM_MMIO);
>
> I believe, after:
>
> f6106d41ec84 ("x86/bugs: Use an x86 feature to track the MMIO Stale Data
> mitigation")
>

Ack.  Actually It looks like I need to clear X86_FEATURE_CLEAR_CPU_BUF, X86_FEATURE_CLEAR_CPU_VM, and the one you noted.

(Although technically those first two do get cleared by other reset functions).

Thanks
--David Kaplan