linux-kernel - Re: [PATCH 4/9] x86/alternative: Implement .retpoline

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Message-ID: <YWdZbdo1fVd7JbBA@hirez.programming.kicks-ass.net>
Date:   Thu, 14 Oct 2021 00:10:53 +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 4/9] x86/alternative: Implement .retpoline_sites support

On Wed, Oct 13, 2021 at 02:49:07PM -0700, Josh Poimboeuf wrote:
> So you've got a WARN_ON_ONCE, a BUG_ON, and a too-deep feature check,
> all in the name of supporting this scheme.  ok...

Mostly because I'm way too lazy to type out that table. With the thunks
being in .S, I really don't see that getting messed up, they even in
their own special section too.

> If performance of the linear lookup were a real concern then you could
> just put rax and r11 at the beginning of the array.

That would mean the table would have to be { __thunk, reg_idx }, which
is even more yuck.