linux-kernel - Re: [PATCH RESEND v2 3/3] x86/cpu: Enable modifying bug flags with {clear,set}puid

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Message-ID: <CA+i-1C0=tDMpfZqNq0aWns=cj70UOOmCAPOonmJi+MM7B6G9Kg@mail.gmail.com>
Date: Mon, 17 Feb 2025 18:20:33 +0100
From: Brendan Jackman <jackmanb@...gle.com>
To: Borislav Petkov <bp@...en8.de>
Cc: Jonathan Corbet <corbet@....net>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Peter Zijlstra <peterz@...radead.org>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH RESEND v2 3/3] x86/cpu: Enable modifying bug flags with {clear,set}puid

On Mon, 17 Feb 2025 at 18:08, Borislav Petkov <bp@...en8.de> wrote:
>
> On Mon, Feb 17, 2025 at 05:56:32PM +0100, Brendan Jackman wrote:
> > Er, hold on, what chunk can be whacked? Do you mean delete the ability
> > to set clearcpuid by number? There are still features with no name.
>
> Really, which ones?

I just mean the ones without a "name" in cpufeatures.h, e.g.

#define X86_FEATURE_MSR_SPEC_CTRL ( 7*32+16) /* MSR SPEC_CTRL is implemented */

> Are you saying you want to turn off *arbitrary* features? Not only what gets
> advertized in /proc/cpuinfo?

You can already clear arbitrary ones with clearcpuid.

But for bugs, they all have a name. I was thinking that this was
because they are defined by the kernel, that's what I meant by "It t
doesn't make sense for a bug not to have a name", although now I think
about it we could totally have a bug and not give it a user-visible
name if we wanted to.

Anyway, still think the current logic is what we want here:

- The new setcpuid should be consistent with the existing clearcpuid,
i.e. accept numbers for the same things clearcpuid does.

- There are currently no bugs without names so for those, require the
string for both setcpuid and clearcpuid. If we wanted to we could add
number support later.