linux-kernel - Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

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Message-ID: <a035392c-f332-4b3f-b851-13b0c7a0fc68@email.android.com>
Date:	Mon, 21 Apr 2014 18:14:54 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andrew Lutomirski <amluto@...il.com>
CC:	"H. Peter Anvin" <hpa@...ux.intel.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Alexander van Heukelum <heukelum@...tmail.fm>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	Borislav Petkov <bp@...en8.de>,
	Arjan van de Ven <arjan.van.de.ven@...el.com>,
	Brian Gerst <brgerst@...il.com>,
	Alexandre Julliard <julliard@...ehq.com>,
	Andi Kleen <andi@...stfloor.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] x86-64: espfix for 64-bit mode *PROTOTYPE*

I wanted to avoid the "another cpu made this allocation, now I have to free" crap, but I also didn't want to grab the lock if there was no work needed.

On April 21, 2014 6:06:19 PM PDT, Andrew Lutomirski <amluto@...il.com> wrote:
>On Mon, Apr 21, 2014 at 5:53 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> Well, if 2^17 CPUs are allocated we might 2K pages allocated.  We
>could easily do a bitmap here, of course.  NR_CPUS/64 is a small
>number, and would reduce the code complexity.
>>
>
>Even simpler: just get rid of the check entirely.  That is, break out
>of the higher level loops once one of them is set (this should be a
>big speedup regardless) and don't allocate the page if the first PTE
>is already pointing at something.
>
>After all, espfix_already_there is mostly a duplicate of
>init_espfix_cpu.
>
>--Andy

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