From: Borislav Petkov <bp@alien8.de>
To: Andy Lutomirski <luto@amacapital.net>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Brian Gerst <brgerst@gmail.com>,
X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 6/6] x86/vdso: Use static_cpu_has()
Date: Mon, 25 Jan 2016 19:58:50 +0100 [thread overview]
Message-ID: <20160125185850.GD14030@pd.tnic> (raw)
In-Reply-To: <CALCETrUt42BNJEY+dskCKF8Tmwu0keMo0L-faxefMKBcKRGRFg@mail.gmail.com>
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.
prev parent reply other threads:[~2016-01-25 18:59 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-01-24 9:28 [PATCH 0/6] x86/cpufeature: Cleanups and improvements v1 Borislav Petkov
2016-01-24 9:28 ` [PATCH 1/6] x86/cpufeature: Carve out X86_FEATURE_* Borislav Petkov
2016-01-24 9:28 ` [PATCH 2/6] x86/cpufeature: Replace the old static_cpu_has() with safe variant Borislav Petkov
2016-01-24 9:28 ` [PATCH 3/6] x86/cpufeature: Get rid of the non-asm goto variant Borislav Petkov
2016-01-24 9:28 ` [PATCH 4/6] x86/alternatives: Add an auxilary section Borislav Petkov
2016-01-24 9:28 ` [PATCH 5/6] x86/alternatives: Discard dynamic check after init Borislav Petkov
2016-01-24 9:28 ` [PATCH 6/6] x86/vdso: Use static_cpu_has() Borislav Petkov
2016-01-25 18:45 ` Andy Lutomirski
2016-01-25 18:58 ` Borislav Petkov [this message]
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