linux-kernel - Re: Retbleed, Zen2 and STIBP

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Message-ID: <YtXb7UF4JVh3GzIG@llamedos.localdomain>
Date:   Mon, 18 Jul 2022 23:17:17 +0100
From:   Ken Moffat <zarniwhoop@...world.com>
To:     linux-kernel@...r.kernel.org
Subject: Re: Retbleed, Zen2 and STIBP

On Mon, Jul 18, 2022 at 09:19:01AM +0100, Ken Moffat wrote:

Replying to myself to close this speculative misunderstanding.

> 
> While ibpb might be available (and slow), on my Renoir with
> microcode level (0860106h) there were no newer microcode versions
> available when I last looked (a few weeks ago) but note 7 at the
> bottom of
> https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v
> 7_20220712.pdf
> implies that the relevant bit is only set on Renoir in 0860109h and
> later.
> 
> Some of the text in that pdf implies that at least one of the
> options could be set if not already set from the microcode, but the
> amount of detail leaves me totally lost.
> 

Now that I've tried the options, I can see that ibpb *is* available
on this zen2 desktop machine (and no, I have no desire to use it,
having seen how much things slow down).  The default (auto) on this
zen2 is the same as 'unret', i.e.

Mitigation: untrained return thunk; SMT enabled with STIBP protection

ĸen
-- 
 It is very easy to get ridiculously confused about the tenses of
 time travel, but most things can be resolved by a sufficiently
 large ego.        -- The Last Continent