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

[Ben Hutchings]

On Fri, 2022-08-19 at 10:47 +0200, Peter Zijlstra wrote:
> 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 ?!?

Yes, X86_FEATURE_RSB_CTXSW is enabled if any other Spectre v2
mitigation is enabled.  And all Intel family 6 (except some early
Atoms) and AMD family 5+ get Spectre v2 mitigation by default.

Ben.

-- 
Ben Hutchings
Beware of bugs in the above code;
I have only proved it correct, not tried it. - Donald Knuth

Attachment: signature.asc
Description: This is a digitally signed message part