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

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Message-ID: <ee12ff5e-91fe-487b-bed9-4472f15f94fe@email.android.com>
Date:	Mon, 21 Apr 2014 18:47:52 -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*

Race condition (although with x86 being globally ordered, it probably can't actually happen.) The bitmask is probably the way to go.

On April 21, 2014 6:28:12 PM PDT, Andrew Lutomirski <amluto@...il.com> wrote:
>On Mon, Apr 21, 2014 at 6:14 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> 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.
>
>I guess you also want to avoid bouncing all these cachelines around on
>boot on bit multicore machines.
>
>I'd advocate using the bitmap approach or simplifying the existing
>code.  For example:
>
>+       for (n = 0; n < ESPFIX_PUD_CLONES; n++) {
>+               pud = ACCESS_ONCE(pud_p[n]);
>+               if (!pud_present(pud))
>+                       return false;
>+       }
>
>I don't see why that needs to be a loop.  How can one clone exist but
>not the others?
>
>--Andy

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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