netdev - Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable

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Date:	Mon, 19 Mar 2007 22:54:20 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Andi Kleen <ak@....de>, David Miller <davem@...emloft.net>,
	mingo@...e.hu, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, virtualization@...ts.osdl.org,
	xen-devel@...ts.xensource.com, chrisw@...s-sol.org,
	anthony@...emonkey.ws, netdev@...r.kernel.org
Subject: Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops
 callsites to make them patchable

Zachary Amsden wrote:
> For VMI, the default clobber was "cc", and you need a way to allow at
> least that, because saving and restoring flags is too expensive on x86.

According to lore (Andi, I think), asm() always clobbers cc. 

> I still don't think this was a good trade.  The primary motivation for
> clobbering %eax was that Xen wanted a free register to use for
> computing the offset into the shared data in the case of SMP
> preemptible kernels.  Xen no longer needs such a register, they can
> use the PDA offset instead.  And it does hurt native performance by
> unconditionally stealing a register in the four most commonly invoked
> paravirt-ops code sequences.

Actually, it still does need a temp register.  The sequence for cli is:

    mov %fs:xen_vcpu, %eax
    movb $1,1(%eax)

At some point I hope to move the vcpu structure directly into the
pda/percpu variables, at which point it will need no temps.

    J
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