linux-kernel - Re: [PATCH 03/23] x86/srso: Don't probe microcode in a guest

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Message-ID: <cc4f4937-c696-cb32-939d-bbeafc4210b5@citrix.com>
Date:   Fri, 25 Aug 2023 08:52:38 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Josh Poimboeuf <jpoimboe@...nel.org>, x86@...nel.org
Cc:     linux-kernel@...r.kernel.org, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Babu Moger <babu.moger@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>, David.Kaplan@....com,
        Nikolay Borisov <nik.borisov@...e.com>,
        gregkh@...uxfoundation.org, Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 03/23] x86/srso: Don't probe microcode in a guest

On 25/08/2023 8:01 am, Josh Poimboeuf wrote:
> To support live migration, the hypervisor sets the "lowest common
> denominator" of features.  Probing the microcode isn't allowed because
> any detected features might go away after a migration.
>
> As Andy Cooper states:
>
>   "Linux must not probe microcode when virtualised.  What it may see
>   instantaneously on boot (owing to MSR_PRED_CMD being fully passed
>   through) is not accurate for the lifetime of the VM."
>
> Rely on the hypervisor to set the needed IBPB_BRTYPE and SBPB bits.
>
> Fixes: 1b5277c0ea0b ("x86/srso: Add SRSO_NO support")
> Suggested-by: Andrew Cooper <andrew.cooper3@...rix.com>
> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>

Reviewed-by: Andrew Cooper <andrew.cooper3@...rix.com>

Thankyou for doing this patch.