linux-kernel - Re: [PATCH] x86/srso: Explain the untraining sequences a bit more

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Message-ID: <acd17a36-ea03-3db7-5333-d920bc65cb8a@suse.com>
Date:   Tue, 15 Aug 2023 18:41:58 +0300
From:   Nikolay Borisov <nik.borisov@...e.com>
To:     Borislav Petkov <bp@...en8.de>,
        Josh Poimboeuf <jpoimboe@...nel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org, David.Kaplan@....com,
        Andrew.Cooper3@...rix.com, gregkh@...uxfoundation.org
Subject: Re: [PATCH] x86/srso: Explain the untraining sequences a bit more



On 15.08.23 г. 17:26 ч., Borislav Petkov wrote:
> From: "Borislav Petkov (AMD)" <bp@...en8.de>
> Date: Mon, 14 Aug 2023 21:29:50 +0200
> 
> The goal is to eventually have a proper documentation about all this.
> 
> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
> 
> diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
> index 915c4fe17718..e59c46581bbb 100644
> --- a/arch/x86/lib/retpoline.S
> +++ b/arch/x86/lib/retpoline.S
> @@ -183,6 +183,25 @@ SYM_CODE_START(srso_alias_return_thunk)
>   	ud2
>   SYM_CODE_END(srso_alias_return_thunk)
>   
> +/*
> + * Some generic notes on the untraining sequences:
> + *
> + * They are interchangeable when it comes to flushing potentially wrong
> + * RET predictions from the BTB.
> + *
> + * The SRSO Zen1/2 (MOVABS) untraining sequence is longer than the
> + * Retbleed sequence because the return sequence done there
> + * (srso_safe_ret()) is longer and the return sequence must fully nest
> + * (end before) the untraining sequence. Therefore, the untraining
> + * sequence must fully overlap the return sequence.
> + *
> + * Regarding alignment - the instructions which need to be untrained,
> + * must all start at a cacheline boundary for Zen1/2 generations. That
> + * is, instruction sequences starting at srso_safe_ret() and
> + * the respective instruction sequences at retbleed_return_thunk()
> + * must start at a cacheline boundary.
> + */

Are there any salient generic details about zen 3/4 ?
> +
>   /*
>    * Safety details here pertain to the AMD Zen{1,2} microarchitecture:
>    * 1) The RET at retbleed_return_thunk must be on a 64 byte boundary, for