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

From: Daniel Borkmann
Date: Mon Mar 25 2019 - 10:37:34 EST

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.