linux-kernel - Re: [PATCH 6/6] x86/vdso: Use static_cpu

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Date:	Mon, 25 Jan 2016 19:58:50 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Brian Gerst <brgerst@...il.com>,
	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 6/6] x86/vdso: Use static_cpu_has()

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@...en8.de> wrote:
> > From: Borislav Petkov <bp@...e.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.

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