Re: [Xen-devel] FW: VT-d async invalidation for Device-TLB.

["Xu, Quan" <quan.xu@xxxxxxxxx>]

>On June 18, 2015 5:19 PM, <JBeulich@xxxxxxxx> wrote:
> >>> On 18.06.15 at 10:09, <quan.xu@xxxxxxxxx> wrote:
> 
> >> On 16.06.15 09:59, <mailto:JBeulich@xxxxxxxx> wrote:
> >> >>> On 16.06.15 at 09:59, <yang.z.zhang@xxxxxxxxx> wrote:
> >> > Jan Beulich wrote on 2015-06-16:
> >> >>>>> On 16.06.15 at 05:07, <yang.z.zhang@xxxxxxxxx> wrote:
> >> >>> Jan Beulich wrote on 2015-06-15:
> >> >>>>>>> On 13.06.15 at 16:44, <quan.xu@xxxxxxxxx> wrote:
> >> >>>>>> On 12.06.15 at 14:47, <JBeulich@xxxxxxxx> wrote:
> >> >>>>>>>>> On 12.06.15 at 04:40, <quan.xu@xxxxxxxxx> wrote:
> >
> >> >
> >> > which one? 1.4us for sync case and 4.3us for async case?
> >>
> >> The difference between the two (i.e. why is the async variant three
> >> times as long).
> >>
> >
> > I have tested iotlb async/sync invalidation to get another data. Also
> > I disabled 'P State' / 'C State' in bios.
> > For async invalidation: about 2.67 us.
> > For sync invalidation: about 1.28 us.
> >
> > I also tested VCPU_KICK_SOFTIRQ irq cost.
> >  When hypervisor calls cpu_raise_softirq(..VCPU_KICK_SOFTIRQ) to raise
> > an VCPU_KICK_SOFTIRQ irq, and vcpu_kick_softirq() is the
> > VCPU_KICK_SOFTIRQ interrupt handler.
> > I got the cost between cpu_raise_softirq(..VCPU_KICK_SOFTIRQ) and
> > vcpu_kick_softirq(). It is about 1.21 us.
> >
> > I think the difference is interrupt cost between async invalidation
> > and sync invalidation.
> 
> Which contradicts what I think Yang said in an earlier reply.
> 
Talked with Yang who is confused at what he said. :(
Could you share more?


> > 2.67us is almost ideal for async invalidation cost. There are 4
> > reasons to cost much more time:
> >    1.  If enable 'P State' / 'C State' in bios.
> >    2.  Hypervisor is running in No-root mode.
> >    3.  The time doesn't include the cost of handling of interrupt. I
> > just record it at the entry of interrupt handler.
> >    4.  More pass-through VMs runs.
> >
> > So there are maybe some performance issues when we replace the current
> > spinning for the non-ATS case.
> > We can start from ATS case firstly, And apply it to non-ATS case later
> > if the async approach performance is acceptable.
> > Jan, Do you agree with this?
> 
> No, I'm still not convinced that leaving the non-ATS case alone initially is 
> the right
> approach. But maybe I'm the only one?
> 
I hope for someone else to give some comments.

I tried to replace the current spinning for the non-ATS case, but Xen crashed.
Based on dmesg, it seems that VT-d is enabled before enabling IO-APIC IRQs. 

I can send out two serious of patch:
1st: VT-d async invalidation for ATS case.
2nd: VT-d async invalidation for non-ATS case.


I think the 1st serious of patch is high priority, as it is not correct to spin 
1 second for ATS case. I can implement these source code and send out ASAP.
2nd serious of patch is low priority, as it's optimization. Also I can provide 
a serious of patch to fix it later.
Agree?

Quan


> Jan
> 
> 
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