linux-kernel - Re: [RFC PATCH 0/3] generic hypercall support

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Date:	Thu, 7 May 2009 20:35:03 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	Chris Wright <chrisw@...s-sol.org>,
	Gregory Haskins <gregory.haskins@...il.com>,
	Avi Kivity <avi@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org, Anthony Liguori <anthony@...emonkey.ws>
Subject: Re: [RFC PATCH 0/3] generic hypercall support

On Thu, May 07, 2009 at 01:03:45PM -0400, Gregory Haskins wrote:
> Chris Wright wrote:
> > * Gregory Haskins (ghaskins@...ell.com) wrote:
> >   
> >> Chris Wright wrote:
> >>     
> >>> VF drivers can also have this issue (and typically use mmio).
> >>> I at least have a better idea what your proposal is, thanks for
> >>> explanation.  Are you able to demonstrate concrete benefit with it yet
> >>> (improved latency numbers for example)?
> >>>       
> >> I had a test-harness/numbers for this kind of thing, but its a bit
> >> crufty since its from ~1.5 years ago.  I will dig it up, update it, and
> >> generate/post new numbers.
> >>     
> >
> > That would be useful, because I keep coming back to pio and shared
> > page(s) when think of why not to do this.  Seems I'm not alone in that.
> >
> > thanks,
> > -chris
> >   
> 
> I completed the resurrection of the test and wrote up a little wiki on
> the subject, which you can find here:
> 
> http://developer.novell.com/wiki/index.php/WhyHypercalls
> 
> Hopefully this answers Chris' "show me the numbers" and Anthony's "Why
> reinvent the wheel?" questions.
> 
> I will include this information when I publish the updated v2 series
> with the s/hypercall/dynhc changes.
> 
> Let me know if you have any questions.

Greg,

I think comparison is not entirely fair. You're using
KVM_HC_VAPIC_POLL_IRQ ("null" hypercall) and the compiler optimizes that
(on Intel) to only one register read:

        nr = kvm_register_read(vcpu, VCPU_REGS_RAX);

Whereas in a real hypercall for (say) PIO you would need the address,
size, direction and data.

Also for PIO/MMIO you're adding this unoptimized lookup to the 
measurement:

        pio_dev = vcpu_find_pio_dev(vcpu, port, size, !in);
        if (pio_dev) {
                kernel_pio(pio_dev, vcpu, vcpu->arch.pio_data);
                complete_pio(vcpu); 
                return 1;
        }

Whereas for hypercall measurement you don't. I believe a fair comparison
would be have a shared guest/host memory area where you store guest/host
TSC values and then do, on guest:

	rdtscll(&shared_area->guest_tsc);
	pio/mmio/hypercall
	... back to host
	rdtscll(&shared_area->host_tsc);

And then calculate the difference (minus guests TSC_OFFSET of course)?

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