linux-kernel - Retbleed, Zen2 and STIBP

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Date:   Mon, 18 Jul 2022 09:19:01 +0100
From:   Ken Moffat <zarniwhoop@...world.com>
To:     linux-kernel@...r.kernel.org
Subject: Retbleed, Zen2 and STIBP

Probably like most people, I find the detail of the available
retbleed mitigations obscure.  In particular, for zen2 the options
*might* include ibpb or unret.

But I have failed to find what 'unret' actually means.  Any
pointers, please ?

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.

Assuming, for the moment, that I might want to try this full
mitigation, is there any way to set this in the absence of newer
microcode ?

Or should I just accept that the best I can get is 'unret', whatever
that means ?

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