Re: [PATCH] KVM: emulator: do not needlesly sync registers from emulator ctxt to vcpu

[Gleb Natapov <gleb@redhat.com>]

On Wed, Mar 30, 2011 at 02:48:28PM +0200, Gleb Natapov wrote:
> On Wed, Mar 30, 2011 at 02:17:55PM +0200, Avi Kivity wrote:
> > On 03/30/2011 01:43 PM, Gleb Natapov wrote:
> > >After reboot perf started to work. I ran modified emulator.flat unit
> > >test. It was modified to run test_cmps() in an endless loop.
> > >
> > >Without patch:
> > >1.71%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
> > >1.51%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
> > >1.68%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
> > >
> > >With patch:
> > >0.84%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
> > >0.96%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
> > >0.89%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
> > >
> > 
> > The cause might be kvm_rip_write() using vmwrite.  Can you use perf
> > to see where the hits are in x86_emulate_instruction?
> > 
> > If that's the case, we may be able to do local optimizations to
> > kvm_rip_write(), kvm_set_rflags(), and toggle_interruptiblity()
> > instead of this global change.
> > 
> I can leave copying there and eliminate only kvm_rip_write and see
> perf data.
> 

1.75%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
1.60%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction
1.42%  qemu-system-x86  [kvm]                 [k] x86_emulate_instruction

This is with copy in place, but those are under if (writeback):
                toggle_interruptibility(vcpu,
                                vcpu->arch.emulate_ctxt.interruptibility);
                kvm_set_rflags(vcpu, vcpu->arch.emulate_ctxt.eflags);
                kvm_make_request(KVM_REQ_EVENT, vcpu);
                vcpu->arch.emulate_regs_need_sync_to_vcpu = false;
                kvm_rip_write(vcpu, vcpu->arch.emulate_ctxt.eip);

--
			Gleb.