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Message-ID: <45FF770C.7050301@goop.org>
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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