linux-kernel - Re: [PATCH] x86, retpolines: entirely disable switch jump tables when retpolines are enabled

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Message-ID: <d7965189-c984-b177-8c69-c0e7ee78e71f@iogearbox.net>
Date:   Mon, 25 Mar 2019 15:37:25 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     David Woodhouse <dwmw2@...radead.org>, tglx@...utronix.de
Cc:     linux-kernel@...r.kernel.org, mliska@...e.cz,
        "H . J . Lu" <hjl.tools@...il.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        "David S . Miller" <davem@...emloft.net>
Subject: Re: [PATCH] x86, retpolines: entirely disable switch jump tables when
 retpolines are enabled

On 03/25/2019 03:28 PM, David Woodhouse wrote:
> On Mon, 2019-03-25 at 14:56 +0100, Daniel Borkmann wrote:
>> More than 20 switch cases are not expected to be fast-path critical, but
>> it would still be good to align with gcc behavior for versions < 8.4.0 in
>> order to have consistency across supported gcc versions. vmlinux size is
>> slightly growing by 0.27% for older gcc. This flag is only set to work
>> around affected gcc, no change for clang.
> 
> I note your final sentence doesn't actually say that clang doesn't have
> this problem, and doesn't *need* this (or an equivalent) change.
> 
> It should say that (if it's true). And if it isn't true, then other
> remedial action would be in order.

clang doesn't have this problem as analyzed back in ce02ef06fcf7 ("x86,
retpolines: Raise limit for generating indirect calls from switch-case").

I thought both here would make it quite clear, from this patch commit msg:

"After this has been brought to attention to gcc folks [0], Martin Liska
 has then fixed gcc to align with clang by avoiding to generate switch
 jump tables entirely under retpolines."

And the comment in the Makefile code:

  # Additionally, avoid generating expensive indirect jumps which
  # are subject to retpolines for small number of switch cases.
  # clang turns off jump table generation by default when under
  # retpoline builds, however, gcc does not for x86. This has
  # only been fixed starting from gcc stable version 8.4.0 and
  # onwards, but not for older ones. See gcc bug #86952.

Thanks,
Daniel