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Date:   Mon, 18 Jul 2022 16:58:49 +0200
From:   Alexandre Chartre <alexandre.chartre@oracle.com>
To:     Ken Moffat <zarniwhoop@ntlworld.com>
Cc:     linux-kernel@vger.kernel.org
Subject: Re: Retbleed, Zen2 and STIBP
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On Mon, Jul 18, 2022 at 09:19:01AM +0100, Ken Moffat wrote:
> 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

'unret' = AMD JMP2RET i.e. replace all 'ret' instructions with
'jmp __x86_return_thunk', and safe training the thunk code upon
kernel/hypervisor entry. This is a purely software mitigation,
it doesn't require any microcode.

AMD JMP2RET is described in this document:
https://www.amd.com/system/files/documents/technical-guidance-for-mitigating-branch-type-confusion_v7_20220712.pdf

alex.