From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable Date: Mon, 19 Mar 2007 22:54:20 -0700 Message-ID: <45FF770C.7050301@goop.org> References: <20070316.023331.59468179.davem@davemloft.net> <45FB005D.9060809@goop.org> <1174127638.8897.75.camel@localhost.localdomain> <20070318.003309.71088169.davem@davemloft.net> <20070318120814.GA45869@muc.de> <1174272469.11680.23.camel@localhost.localdomain> <1174348905.11680.54.camel@localhost.localdomain> <45FF4043.4000805@vmware.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <45FF4043.4000805@vmware.com> Sender: netdev-owner@vger.kernel.org To: Zachary Amsden Cc: Rusty Russell , Linus Torvalds , "Eric W. Biederman" , Andi Kleen , David Miller , mingo@elte.hu, akpm@linux-foundation.org, linux-kernel@vger.kernel.org, virtualization@lists.osdl.org, xen-devel@lists.xensource.com, chrisw@sous-sol.org, anthony@codemonkey.ws, netdev@vger.kernel.org List-Id: virtualization@lists.linuxfoundation.org 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