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SubjectRe: [RFC] x86/cpu_entry_area: move part of it back to fixmap
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at 7:11 AM, Andy Lutomirski <luto@amacapital.net> wrote:

>
>
> On Oct 3, 2018, at 9:59 PM, Nadav Amit <namit@vmware.com> wrote:
>
>> This RFC proposes to return part of the entry-area back to the fixmap to
>> improve system-call performance. Currently, since the entry-area is
>> mapped far (more than 2GB) away from the kernel text, an indirect branch
>> is needed to jump from the trampoline into the kernel. Due to Spectre
>> v2, vulnerable CPUs need to use a retpoline, which introduces an
>> overhead of >20 cycles.
>
> That retpoline is gone in -tip. Can you see how your code stacks up against -tip? If it’s enough of a win to justify the added complexity, we can try it.
>
> You can see some pros and cons in the changelog:
>
> https://git.kernel.org/tip/bf904d2762ee6fc1e4acfcb0772bbfb4a27ad8a6

Err.. That’s what I get for not following lkml. Very nice discussion.
Based on it, I may be able to do an additional micro-optimizations or
two. Let me give it a try.

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