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

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Message-ID: <20230816083057.GCZNyJQVARsxD3rNAm@fat_crate.local>
Date:   Wed, 16 Aug 2023 10:30:57 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Josh Poimboeuf <jpoimboe@...nel.org>
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 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.

* 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.

* Now X86_FEATURE_SRSO_NO is passed in by KVM to the guest and the above
dance repeats.

Now you.

-- 
Regards/Gruss,
    Boris.

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