From: Borislav Petkov <bp@alien8.de>
To: Brian Gerst <brgerst@gmail.com>
Cc: X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 1/6] x86/fpu/regset: Use boot_cpu_has()
Date: Tue, 5 Apr 2016 17:11:56 +0200 [thread overview]
Message-ID: <20160405151156.GE17541@pd.tnic> (raw)
In-Reply-To: <CAMzpN2iHYs1UO6RCXRQg0g6pvgRCCP9MQ-at0fTELvVAhbJ6Jw@mail.gmail.com>
On Tue, Apr 05, 2016 at 09:51:55AM -0400, Brian Gerst wrote:
> I think the general rule should be always use static_cpu_has() for
> runtime checks, since it reduces down to a single jmp/nop instruction
> after alternatives run. Even if it's not a hot path, it saves a bit
> of runtime memory.
So gcc generates a MOV and a TEST for boot_cpu_has(). Not exactly the
bloat or slowdown.
So I look at those differently: static_cpu_has()
a) should be used on hotpaths
b) is used as such to annotate those - i.e., "ah, there's a
static_cpu_has(), this is a hot path, I better be careful what I'm doing
here."
boot_cpu_has() in all the rest.
And saving a couple of bytes per call site but increasing alternatives
application time by milli- or microseconds... Meh. I don't see it being
really worth the trouble. But this is just me.
--
Regards/Gruss,
Boris.
ECO tip #101: Trim your mails when you reply.
next prev parent reply other threads:[~2016-04-05 15:12 UTC|newest]
Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-04-05 6:29 [PATCH 0/6] x86: Misc cleanups Borislav Petkov
2016-04-05 6:29 ` [PATCH 1/6] x86/fpu/regset: Use boot_cpu_has() Borislav Petkov
2016-04-05 13:51 ` Brian Gerst
2016-04-05 15:11 ` Borislav Petkov [this message]
2016-04-13 11:41 ` [tip:x86/asm] x86/fpu/regset: Replace static_cpu_has() usage with boot_cpu_has() tip-bot for Borislav Petkov
2016-04-05 6:29 ` [PATCH 2/6] x86/cpu: Simplify extended APIC ID detection on AMD Borislav Petkov
2016-04-13 11:41 ` [tip:x86/asm] " tip-bot for Borislav Petkov
2016-04-05 6:29 ` [PATCH 3/6] x86/tsc: Do not check X86_FEATURE_CONSTANT_TSC in notifier call Borislav Petkov
2016-04-05 8:00 ` Peter Zijlstra
2016-04-13 11:41 ` [tip:x86/asm] " tip-bot for Borislav Petkov
2016-04-05 6:29 ` [PATCH 4/6] x86/tsc: Save an indentation level in recalibrate_cpu_khz() Borislav Petkov
2016-04-05 8:05 ` Peter Zijlstra
2016-04-13 11:42 ` [tip:x86/asm] " tip-bot for Borislav Petkov
2016-04-05 6:29 ` [PATCH 5/6] x86/fpu: Remove check_fpu() indirection Borislav Petkov
2016-04-13 11:42 ` [tip:x86/asm] " tip-bot for Borislav Petkov
2016-04-05 6:29 ` [PATCH 6/6] x86/fpu: Get rid of x87 math exception helpers Borislav Petkov
2016-04-13 11:42 ` [tip:x86/asm] " tip-bot for Borislav Petkov
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