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

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Message-ID: <CAPZXPQeYh_BrZzinsvCjHvd=szAsOXUmkVYS1tJC5vwamx+Wow@mail.gmail.com>
Date:   Tue, 30 Aug 2022 14:42:04 +0300
From:   Martin-Éric Racine <martin-eric.racine@....fi>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ben Hutchings <ben@...adent.org.uk>, x86@...nel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        1017425@...s.debian.org, 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

Greetings,

On Fri, Aug 19, 2022 at 3:15 PM Peter Zijlstra <peterz@...radead.org> wrote:
>
> On Fri, Aug 19, 2022 at 01:38:27PM +0200, Ben Hutchings wrote:
>
> > So that puts the whole __FILL_RETURN_BUFFER inside an alternative, and
> > we can't have nested alternatives.  That's unfortunate.
>
> Well, both alternatives end with the LFENCE instruction, so I could pull
> it out and do two consequtive ALTs, but unrolling the loop for i386 is
> a better solution in that the sequence, while larger, removes the need
> for the LFENCE.

Have we reached a definitive conclusion on to how to fix this?

Martin-Éric