linux-kernel - Re: [PATCH] x86/bugs: Do not enable IBPB at firmware entry when IBPB is not available

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Date:   Thu, 28 Jul 2022 09:39:20 -0300
From:   Thadeu Lima de Souza Cascardo <cascardo@...onical.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Dimitri John Ledkov <dimitri.ledkov@...onical.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/bugs: Do not enable IBPB at firmware entry when IBPB
 is not available

On Thu, Jul 28, 2022 at 02:35:48PM +0200, Borislav Petkov wrote:
> drop stable@
> 
> On Thu, Jul 28, 2022 at 09:26:02AM -0300, Thadeu Lima de Souza Cascardo wrote:
> > Some cloud hypervisors do not provide IBPB on very recent CPU processors,
> > including AMD processors affected by Retbleed.
> 
> Which hypervisors are those? How relevant is that use case?

Azure for sure, and I guess AWS would be affected as well.

> 
> How do I reproduce it here?
> 

qemu -cpu host,ibpb=off on an affected AMD processor worked for me. With EFI,
so with something like -drive if=pflash,index=0,file=OVMF_CODE_4M.fd -drive
if=pflash,index=1,file=OVMF_VARS_4M.fd.

Cascardo.

> Thx.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette