linux-kernel - Re: [PATCH] x86/speculation: Avoid LFENCE in FILL_RETURN

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Message-ID: <2d899b4a9a08f79396a071eb8c06d524ae6033b0.camel@decadent.org.uk>
Date:   Fri, 19 Aug 2022 13:02:17 +0200
From:   Ben Hutchings <ben@...adent.org.uk>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        1017425@...s.debian.org,
        Martin-Éric Racine <martin-eric.racine@....fi>,
        stable@...r.kernel.org, regressions@...ts.linux.dev,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Subject: Re: [PATCH] x86/speculation: Avoid LFENCE in FILL_RETURN_BUFFER on
 CPUs that lack it

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@...ian.org>
> > 
> > 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

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