linux-kernel - Re: [PATCH] x86/asm/entry/64: better check for canonical address

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Date:	Fri, 27 Mar 2015 09:57:59 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Denys Vlasenko <dvlasenk@...hat.com>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86/asm/entry/64: better check for canonical address

On Thu, Mar 26, 2015 at 11:45:19AM -0700, Andy Lutomirski wrote:
> I suspect that the two added ALU ops are free for all practical
> purposes, and the performance of this path isn't *that* critical.
> 
> If anyone is running with vsyscall=native because they need the
> performance, then this would be a big win.  Otherwise I don't have a
> real preference.  Anyone else have any thoughts here?
> 
> Let me just run through the math quickly to make sure I believe all the numbers:
> 
> Canonical addresses either start with 17 zeros or 17 ones.
> 
> In the old code, we checked that the top (64-47) = 17 bits were all
> zero.  We did this by shifting right by 47 bits and making sure that
> nothing was left.
> 
> In the new code, we're shifting left by (64 - 48) = 16 bits and then
> signed shifting right by the same amount, this propagating the 17th
> highest bit to all positions to its left.  If we get the same value we
> started with, then we're good to go.
> 
> So it looks okay to me.
> 
> IOW, the new code extends the optimization correctly to one more case
> (native vsyscalls or the really weird corner case of returns to
> emulated vsyscalls, although that should basically never happen) at
> the cost of two probably-free ALU ops.

If we're going to apply this, I'd like this text in the commit message
please.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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