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

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Date:   Mon, 25 Mar 2019 14:43:12 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Daniel Borkmann <daniel@...earbox.net>, 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 Mon, 2019-03-25 at 15:37 +0100, Daniel Borkmann wrote:
> 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.

Hm, yes. That ought to be perfectly sufficient for anyone who can
actually read and is paying enough attention.

Sorry :)

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