Subject: Re: [PATCH 2/7]
From: Rusty Russell <rusty@rustcorp.com.au>
To: Jeremy Fitzhardinge <jeremy@goop.org>
Cc: Andi Kleen <ak@muc.de>,
       lkml - Kernel Mailing List <linux-kernel@vger.kernel.org>,
       virtualization <virtualization@lists.osdl.org>
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Date: Sat, 23 Sep 2006 14:36:01 +1000
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On Fri, 2006-09-22 at 15:24 -0700, Jeremy Fitzhardinge wrote:
> Rusty Russell wrote:
> > This patch implements save/restore of %gs in the kernel, so it can be
> > used for per-cpu data.  This is not cheap, and we do it for UP as well
> > as SMP, which is stupid.  Benchmarks, anyone?
> >   
> I measured the cost as adding 9 cycles to a null syscall on my Core Duo 
> machine.  I have not explicitly measured it on other machines, but I run 
> a number of other segment save/load tests on a wide range of machines, 
> and didn't find much variability.

Oh, OK!  I had a belief that segment loading was expensive, perhaps I'm
off-base here.

> I think saving/restoring %gs will still be necessary. There are a number 
> of places in the kernel which expect to find the usermode %gs on the 
> kernel stack frame, including context switch, ptrace, vm86, signal 
> context, and maybe something else.  If you don't save it on the stack, 
> then you need to have UP variations of %gs handling in all those other 
> places, which is pretty messy.  Also, unless you want to have two 
> definitions of struct_pt regs (which would add even more mess into 
> ptrace), you'd still need to sub/add %esp in entry.S to skip over the 
> %gs hole.  I don't think this UP microoptimisation would be worth enough 
> to justify the mess it would cause elsewhere.
> 
> How does this version of the patch differ from mine?  Is it just my 
> patch+Ingo's fix, or are there other changes?  I couldn't see anything 
> from a quick read-over.

Yep, no substative changes.  s/__KERNEL_PDA/__KERNEL_PERCPU/, plus your
version had a "write_pda(pcurrent, next_p)" inserted in process.c's
__switch_to which belonged in a successor patch...

Thanks!
Rusty.
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