Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core logic handling

["Wu, Feng" <feng.wu@xxxxxxxxx>]

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Thursday, September 24, 2015 3:52 PM
> To: Wu, Feng
> Cc: Andrew Cooper; Dario Faggioli; George Dunlap; George Dunlap; Tian, Kevin;
> xen-devel@xxxxxxxxxxxxx; Keir Fraser
> Subject: RE: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core 
> logic
> handling
> 
> >>> On 24.09.15 at 03:50, <feng.wu@xxxxxxxxx> wrote:
> > One issue is the number of vmexits is far far bigger than the number of
> > context switch. I test it for a quite short time and it shows there are
> > 2910043 vmexits and 71733 context switch (only count the number in
> > __context_switch() since we only change the PI state in this function).
> > If we change the PI state in each vmexit/vmentry, I am afraid this will
> > hurt the performance.
> 
> Note that George has already asked whether the updating of the
> PI descriptor is expensive, without you answering. 

Updating the PI descriptor needs to be atomic, I think it should be a little
expensive.

> If this is basically
> just a memory or VMCS field write, I don't think it really matters in
> which code path it sits, regardless of the frequency of either path
> being used. Also note that whatever measuring you do in an area
> like this, it'll only be an example, 

I DON'T think it is just an example, the vmexit numbers is definitely
far larger than context switch.

> unlikely to be representative of anything. 

I don't think so!

> Plus with the tendency to eliminate VMEXITs with newer
> hardware, the penalty of this sitting in the VMEXIT path ought to go
> down.

Broadwell is really very new hardware, the VMEXITs and the number
of context switch is not in the same order of magnitudes.

Thanks,
Feng

> 
> Jan
> 


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