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

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Message-ID: <9395338630e3313c1bf0393ae507925d1f9af870.camel@decadent.org.uk>
Date:   Fri, 19 Aug 2022 13:38:27 +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 13:01 +0200, Peter Zijlstra wrote:
> On Fri, Aug 19, 2022 at 10:47:21AM +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 ?!?
> 
> So I gave in and clicked on the horrible bugzilla thing. Apparently this
> is P3/Athlon64 era crud.
> 
> Anyway, the added LFENCE isn't because of retbleed; it is because you
> can steer the jnz and terminate the loop early and then not actually
> complete the RSB stuffing.

I know, I corrected that further down.

> New insights etc.. So it's a geniune fix for the existing rsb stuffing.
> 
> I'm not entirly sure what to do here. On the one hand, it's 32bit, so
> who gives a crap, otoh we shouldn't break these ancient chips either I
> suppose.
> 
> How's something like so then? It goes on top of my other patch cleaning
> up this RSB mess:
> 
>   https://lkml.kernel.org/r/Yv9m%2FhuNJLuyviIn%40worktop.programming.kicks-ass.net
[...]

That should be:
https://lore.kernel.org/all/Yv9m%2FhuNJLuyviIn@worktop.programming.kicks-ass.net/
(the redirector unescapes the URL-escaped /).

So that puts the whole __FILL_RETURN_BUFFER inside an alternative, and
we can't have nested alternatives.  That's unfortunate.

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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