Re: [Xen-devel] [faster-by-no-jump-table] retpoline: disable jump tables

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Nov 20, 2018 at 08:44:31AM -0700, Jan Beulich wrote:
> >>> On 20.11.18 at 16:37, <nmanthey@xxxxxxxxx> wrote:
> > To mitigate Meltdown, Xen has been fixed with a software fix, namely
> > using retpoline sequences generated by the compiler. This way, indirect
> > branches are protected against the attack.
> > 
> > However, the retpoline sequence comes with a slow down. To make up for
> > this, we propose to avoid jump tables in the first place. Without the
> > retpoline sequences, this code would be less efficient. However, when
> > retpoline is enabled, this actually results in a slight performance
> > improvement.
> > 
> > This change might become irrelevant once the compiler starts avoiding
> > jump tables in case retpolines are used:
> > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86952 
> > 
> > Reported-by: Julian Stecklina <jsteckli@xxxxxxxxx>
> > Reported-by: Pawel Wieczorkiewicz <wipawel@xxxxxxxxx>
> > Signed-off-by: Norbert Manthey <nmanthey@xxxxxxxxx>

Shouldn't this use cc-option-add, or is it guaranteed that every
compiler that supports -mindirect-branch-register will also support
-fno-jump-tables?

Thanks, Roger.

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