linux-kernel - Re: [RFC PATCH 04/56] x86/bugs: Reset spectre

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Message-ID: <839b7bc9-b155-4458-ad36-fa083659283c@intel.com>
Date: Tue, 14 Oct 2025 11:37:56 -0700
From: Dave Hansen <dave.hansen@...el.com>
To: David Kaplan <david.kaplan@....com>, Thomas Gleixner
 <tglx@...utronix.de>, Borislav Petkov <bp@...en8.de>,
 Peter Zijlstra <peterz@...radead.org>, Josh Poimboeuf <jpoimboe@...nel.org>,
 Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
 Ingo Molnar <mingo@...hat.com>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H . Peter Anvin" <hpa@...or.com>
Cc: Alexander Graf <graf@...zon.com>,
 Boris Ostrovsky <boris.ostrovsky@...cle.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 04/56] x86/bugs: Reset spectre_v1 mitigations

On 10/13/25 07:33, David Kaplan wrote:
...
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +static void spectre_v1_reset_mitigation(void)
> +{
> +	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;
> +}
> +#endif
> +
>  static int __init nospectre_v1_cmdline(char *str)
>  {
>  	spectre_v1_mitigation = SPECTRE_V1_MITIGATION_NONE;
> @@ -3794,3 +3805,10 @@ void __warn_thunk(void)
>  {
>  	WARN_ONCE(1, "Unpatched return thunk in use. This should not happen!\n");
>  }
> +
> +#ifdef CONFIG_DYNAMIC_MITIGATIONS
> +void arch_cpu_reset_mitigations(void)
> +{
> +	spectre_v1_reset_mitigation();
> +}
> +#endif

Are all the #ifdefs necessary? For instance, a single:

 void arch_cpu_reset_mitigations(void)
 {
+	if (!IS_ENABLED(CONFIG_DYNAMIC_MITIGATIONS))
+		return;
...

and removing *all* the #ifdefs might be enough to tell the compiler that
all the *_reset_mitigation() functions are unreachable.

But the larger concern through all of this is that this is all *new*
code. The reset concept is completely new and the reset functions look
to be ad-hoc. They follow a few patterns, but nothing super consistent.

Also, this series is going to need to be broken up. I'd suggest going
after the dynamic alternatives, first. I'm sure there's a need for that
_somewhere_ other than for vulnerabilities.

But, in the end, (IMNHO) this series really just adds complexity. It
doesn't do enough refactoring or complexity reduction to go in with this
approach.