Re: [PATCH] x86/speculation: Avoid LFENCE in FILL_RETURN_BUFFER on CPUs that lack it

From: Peter Zijlstra
Date: Fri Aug 19 2022 - 04:47:52 EST


On Fri, Aug 19, 2022 at 02:33:08AM +0200, Ben Hutchings wrote:
> From: Ben Hutchings <benh@xxxxxxxxxx>
>
> The mitigation for PBRSB includes adding LFENCE instructions to the
> RSB filling sequence. However, RSB filling is done on some older CPUs
> that don't support the LFENCE instruction.
>

Wait; what? There are chips that enable the RSB mitigations and DONT
have LFENCE ?!?