linux-kernel - Re: [PATCH] x86/srso: Correct the mitigation status when SMT is disabled

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Message-ID: <20230816160757.oegndrcnf2fvt7l3@treble>
Date:   Wed, 16 Aug 2023 09:07:57 -0700
From:   Josh Poimboeuf <jpoimboe@...nel.org>
To:     Borislav Petkov <bp@...en8.de>
Cc:     Nikolay Borisov <nik.borisov@...e.com>, X86 ML <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/srso: Correct the mitigation status when SMT is
 disabled

On Wed, Aug 16, 2023 at 10:30:57AM +0200, Borislav Petkov wrote:
> On Tue, Aug 15, 2023 at 02:27:51PM -0700, Josh Poimboeuf wrote:
> > How is that relevant to my comment?  The bug bit still wouldn't get set
> > and srso_show_state() still wouldn't be called.
> 
> Lemme explain how I see this working - it might help us get on the right
> track. And for comparison you can look at X86_FEATURE_BTC_NO too.
> 
> * Something has set X86_FEATURE_SRSO_NO - hw or sw doesn't matter
> - because the machine is not affected. X86_BUG_SRSO doesn't get set and
>   the mitigation detection is skipped. All good.

In this case srso_show_state() is never called, so the following code
can't run:

+	if (boot_cpu_has(X86_FEATURE_SRSO_NO)) {
+		if (sched_smt_active())
+			return sysfs_emit(buf, "Not affected\n");

> * Nothing has set X86_FEATURE_SRSO_NO, mitigation detection runs and
> find that the kernel runs on a Zen1/2 with SMT disabled - we set
> X86_FEATURE_SRSO_NO.

In this case SMT is disabled, so the following code still can't run:

+	if (boot_cpu_has(X86_FEATURE_SRSO_NO)) {
+		if (sched_smt_active())
+			return sysfs_emit(buf, "Not affected\n");


So the above code never runs.

See?

-- 
Josh