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

[Dario Faggioli <dario.faggioli@citrix.com>]

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On Thu, 2015-09-24 at 01:50 +0000, Wu, Feng wrote:

> > -----Original Message-----
> > From: George Dunlap [mailto:george.dunlap@citrix.com]

> > This seems to me to be a cleaner design.  It removes the need to
> > modify
> > any code on context-switch path.  It moves modification of NDST and
> > most
> > modifications of SN closer to where I think they logically go.  It
> > reduces the number of unnecessary PI interrupts delivered to the
> > hypervisor (by suppressing notifications most of the time spend in
> > the
> > hypervisor), and automatically deals with the "spurious interrupts
> > during tasklet execution / vcpu offline lazy context switch" issue
> > we
> > were talking about with the other thread.
> 
> 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.
> 
Interesting. Hard to tell without actually trying, though.

Personally, I'd agree with George and Jan that the vmexit solution is
more self contained, and hence preferable.

I don't really dislike the __context_switch() solution, though, and I'd
be fine to live with it, especially considering these numbers.

I guess the absolute best would be for you to prototype both, and try
gathering some performance numbers for comparison... Is this asking too
much? :-)

Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)


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