linux-kernel - Re: [PATCH] Add a text

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Message-ID: <20131127231010.GM32267@pd.tnic>
Date:	Thu, 28 Nov 2013 00:10:10 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andy Lutomirski <luto@...capital.net>,
	Andi Kleen <andi@...stfloor.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andi Kleen <ak@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] Add a text_poke syscall v2

On Wed, Nov 27, 2013 at 02:40:04PM -0800, H. Peter Anvin wrote:
> Also don't forget we need the IPIs, too...

Yeah, I was simply looking at whether we could get away with executing
an empty syscall, i.e. save us the CPUID and rely only on the IPIs and
IRET.

But we don't IPI ourselves in smp_call_function; actually we remove
ourselves from the cpumask because of deadlocking scenarios. So on
this_cpu we only execute the function with IRQs disabled and CLI/STI is
not serializing.

I wonder if we could use MFENCE instead of CPUID, though, and save us
the clobbering of e*x, maybe even save us some cycles since MFENCE
should be faster than hundred-ish cycles of microcoded CPUID.

:-)

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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