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

[Avi Kivity <avi@redhat.com>]

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?

Copying a couple of hot cache lines shouldn't take any measurable time 
compared to a heavyweight exit.

-- 
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