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 03:29:02PM +0200, Avi Kivity wrote:
> On 03/30/2011 03:26 PM, Gleb Natapov wrote:
> >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);
> >
> 
> It's wierd.  Do you get perf hits in the copying?
> 
How can I check. The memcpy is inlined.

> Copying a couple of hot cache lines shouldn't take any measurable
> time compared to a heavyweight exit.
> 
The whole function takes only 1.5% CPU. Perf measures how much this
function become faster and heavyweight exit is not part of the function.

--
			Gleb.