Messages in this thread | | | Date | Mon, 25 Jan 2016 19:58:50 +0100 | From | Borislav Petkov <> | Subject | Re: [PATCH 6/6] x86/vdso: Use static_cpu_has() |
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We discussed this on IRC, here's the gist:
On Mon, Jan 25, 2016 at 10:45:30AM -0800, Andy Lutomirski wrote: > On Sun, Jan 24, 2016 at 1:28 AM, Borislav Petkov <bp@alien8.de> wrote: > > From: Borislav Petkov <bp@suse.de> > > > > ... and simplify and speed up a tad. > > This function is only used when initializing CPUs, so the "tad" is > very small indeed.
... except it'll pay out when the branch is patched in. Considering that the majority of the modern CPUs out there - BSP and APs :-) - have RDTSCP, this check will turn into a 5-byte NOP which is the most optimal we can get. Yeah, it is still an init path so called once on each CPU but still.
> If there are systems for which some cpus support rdtscp and some > don't, then this patch is wrong. Of course, if the bsp has rdtscp and > the aps don't, then we're screwed anyway.
That would be a very odd case.
> I left it as cpu_has because this is a cpu init function and it seemed > reasonable.
Yeah, I see what you mean. But it costs us only the patching and after that we win from not needing for fetch boot_cpu_data anymore on the APs coming up.
Not a panties-dropper speedup but I still think it is worth the trouble.
> That being said, I have no meaningful objection to this patch.
Thanks.
-- Regards/Gruss, Boris.
ECO tip #101: Trim your mails when you reply.
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