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

Jeremy Fitzhardinge
2007-03-20T05:54:26Z

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