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

[Linus Torvalds <torvalds@osdl.org>]

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