linux-kernel - Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL

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Message-ID: <20251007230821.5shpa3pusyzaohb2@desk>
Date: Tue, 7 Oct 2025 16:08:21 -0700
From: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
To: Borislav Petkov <bp@...en8.de>
Cc: "Kaplan, David" <David.Kaplan@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>,
	"H . Peter Anvin" <hpa@...or.com>,
	Klaus Kusche <klaus.kusche@...puterix.info>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/bugs: Qualify RETBLEED_INTEL_MSG

On Wed, Oct 08, 2025 at 12:12:29AM +0200, Borislav Petkov wrote:
> On Tue, Oct 07, 2025 at 11:22:57AM -0700, Pawan Gupta wrote:
> > Even when CPU mitigations are disabled there is still some handling
> > required for mitigations like GDS that requires a write to MSR to ensure
> > correct behavior.
> 
> When mitigations are compiled out there are no mitigations by definition.

GDS microcode mitigation is enabled by default, for disabling the
mitigation an MSR write is required.

> So whatever you're talking about must be something else which should
> happen in vendor code.

Perhaps yes when bugs.c is compiled out.

> > IMO, rather than targeting the mitigation enabling code it might make more
> > sense to compile out the actual mitigations scattered accross the kernel.
> > This may also improve performance by reducing the code footprint, and can
> > also help getting a cleaner disassembly.
> 
> Probably... however, it needs to be done smartly because sprinkling ifdeffery
> and turning what is an already unreadable mess into a bigger abomination,
> won't fly. Perhaps split out the mitigations glue into separate compilation
> units and build-disable them... we'll see.

Ya, that would be better.