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

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Message-ID: <Yv9+8vR4QH6j6J/5@worktop.programming.kicks-ass.net>
Date:   Fri, 19 Aug 2022 14:15:46 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ben Hutchings <ben@...adent.org.uk>
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, 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.