Messages in this thread | | | From | Andy Lutomirski <> | Date | Thu, 4 Oct 2018 10:02:48 -0700 | Subject | Re: [RFC] x86/cpu_entry_area: move part of it back to fixmap |
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On Thu, Oct 4, 2018 at 9:31 AM Nadav Amit <namit@vmware.com> wrote: > > 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. >
I think you should at least try to benchmark your code against mine, since you more or less implemented the alternative I suggested. :)
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