linux-kernel - Re: [rfc patch] i386: don't save eflags on task switch

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Message-ID: <Pine.LNX.4.64.0611050808090.25218@g5.osdl.org>
Date:	Sun, 5 Nov 2006 08:12:30 -0800 (PST)
From:	Linus Torvalds <torvalds@...l.org>
To:	Andi Kleen <ak@...e.de>
cc:	Benjamin LaHaise <bcrl@...ck.org>,
	Zachary Amsden <zach@...are.com>,
	Chuck Ebbert <76306.1226@...puserve.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [rfc patch] i386: don't save eflags on task switch

On Sun, 5 Nov 2006, Andi Kleen wrote:
> 
> This means we should definitely change restore_flags() to only STI, 
> never popf

Whaa? That would be wrong. We don't always sti, quite often the flags were 
disabled anyway.

And changing restore-flags to a "conditional branch around sti" is likely 
not much better - mispredicted branches on a P4 are potentially worse than 
the popf cost.

Side note: for the netburst microarchitecture - aka P4 - in general, 
something like 48 cycles is a _good_ thing. I measured a internal 
micro-fault for marking a page table entry dirty at over 1500 cycles! 
There's a reason Intel dropped Netburst in favour of Core 2 - which is 
largely just an improved Pentium Pro uarch. Admittedly, the "just" is a 
bit unfair, because there's a _lot_ of improvement, but still..

So you should never actually make any real code design decisions based on 
a P4 result. The P4 is goign away, and it was odd. 

		Linus
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