Messages in this thread | | | Date | Mon, 19 Mar 2007 22:54:20 -0700 | From | Jeremy Fitzhardinge <> | Subject | Re: [patch 13/26] Xen-paravirt_ops: Consistently wrap paravirt ops callsites to make them patchable |
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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.
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