RE: [RFC PATCH 05/56] x86/bugs: Reset spectre_v2 mitigations

[Kaplan, David]

[AMD Official Use Only - AMD Internal Distribution Only]

> -----Original Message-----
> From: Borislav Petkov <bp@xxxxxxxxx>
> Sent: Monday, November 3, 2025 1:32 PM
> 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 05/56] x86/bugs: Reset spectre_v2 mitigations
>
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>
> On Mon, Oct 13, 2025 at 09:33:53AM -0500, David Kaplan wrote:
> > Add function to reset spectre_v2 mitigations back to their boot-time
> > defaults.
> >
> > Signed-off-by: David Kaplan <david.kaplan@xxxxxxx>
> > ---
> >  arch/x86/kernel/cpu/bugs.c | 19 +++++++++++++++++++
> >  1 file changed, 19 insertions(+)
> >
> > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> > index 9d5c6a3e50e1..0430635bb17d 100644
> > --- a/arch/x86/kernel/cpu/bugs.c
> > +++ b/arch/x86/kernel/cpu/bugs.c
> > @@ -2477,6 +2477,24 @@ static void __init
> spectre_v2_apply_mitigation(void)
> >       }
> >  }
> >
> > +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> > +static void spectre_v2_reset_mitigation(void)
>
> Yeah, the reset should definitely be a single function - this ifdeffery around
> every single one is going to become yucky.
>

Do you really want it all in one big function?  Or just to relocate all the *_reset_mitigation() functions to a single place so they can all go under one ifdef?

I can do it in one big function, but it'd probably look something like:

/* Reset spectre_v1 */
setup_clear_cpu_cap(X86_FEATURE_FENCE_SWAPGS_USER);
setup_clear_cpu_cap(X86_FEATURE_FENCE_SWAPGS_KERNEL);
spectre_v1_mitigation = SPECTRE_V1_MITIGATION_AUTO;
/* Reset mds */
setup_clear_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
static_branch_disable(&cpu_buf_idle_clear);
mds_mitigation = IS_ENABLED(CONFIG_MITIGATION_MDS) ?
                MDS_MITIGATION_AUTO : MDS_MITIGATION_OFF;
/* Reset spectre_v2 */
Etc.

--David Kaplan