linux-kernel - Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE

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Message-ID: <20211020192341.GQ174703@worktop.programming.kicks-ass.net>
Date:   Wed, 20 Oct 2021 21:23:41 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     x86@...nel.org, andrew.cooper3@...rix.com,
        linux-kernel@...r.kernel.org, alexei.starovoitov@...il.com,
        ndesaulniers@...gle.com
Subject: Re: [PATCH v2 14/14] bpf,x86: Respect X86_FEATURE_RETPOLINE*

On Wed, Oct 20, 2021 at 09:56:55AM -0700, Josh Poimboeuf wrote:
> On Wed, Oct 20, 2021 at 01:09:51PM +0200, Peter Zijlstra wrote:
> > Alexei; could the above not be further improved with something like the
> > below?
> > 
> > Despite several hours trying and Song helping, I can't seem to run
> > anything bpf, that stuff is cursed. So I've no idea if the below
> > actually works, but it seems reasonable.
> 
> The below fix gets it to run with test_verififer.

Argh, I;m fast running out of brown paper bags at this rate :/

> 
> I do like it, it does seem less fiddly and more robust against future
> changes, though it probably needs a comment for 'out_label' clarifying
> it only works because this function is always called at least twice for
> a given bpf_tail_call emission.

Right.