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

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

> 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.

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?


Quan


> Jan


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