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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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>

Acked-by: Jan Beulich <jbeulich@xxxxxxxx>

In the future please play by patch submission rules: Patches get
sent _to_ the list, with maintainers (and whoever else) on _cc_.

Jan



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