Re: [Xen-devel] vmx: VT-d posted-interrupt core logic handling

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

> -----Original Message-----
> From: Konrad Rzeszutek Wilk [mailto:konrad.wilk@xxxxxxxxxx]
> Sent: Tuesday, May 17, 2016 9:27 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; Jan Beulich <JBeulich@xxxxxxxx>;
> Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Dario Faggioli
> <dario.faggioli@xxxxxxxxxx>; David Vrabel <david.vrabel@xxxxxxxxxx>;
> GeorgeDunlap <george.dunlap@xxxxxxxxxx>; Lars Kurth <lars.kurth@xxxxxxxxxx>;
> George Dunlap <George.Dunlap@xxxxxxxxxxxxx>; Ian Jackson
> <Ian.Jackson@xxxxxxxxxxxxx>; xen-devel@xxxxxxxxxxxxx
> Subject: Re: vmx: VT-d posted-interrupt core logic handling
> 
> On Thu, Mar 10, 2016 at 01:36:34PM +0000, Wu, Feng wrote:
> >
> >
> > > -----Original Message-----
> > > From: Tian, Kevin
> > > Sent: Thursday, March 10, 2016 6:06 PM
> > > To: Jan Beulich <JBeulich@xxxxxxxx>
> > > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>; Dario Faggioli
> > > <dario.faggioli@xxxxxxxxxx>; David Vrabel <david.vrabel@xxxxxxxxxx>;
> > > GeorgeDunlap <george.dunlap@xxxxxxxxxx>; Lars Kurth
> <lars.kurth@xxxxxxxxxx>;
> > > George Dunlap <George.Dunlap@xxxxxxxxxxxxx>; Ian Jackson
> > > <Ian.Jackson@xxxxxxxxxxxxx>; Wu, Feng <feng.wu@xxxxxxxxx>; xen-
> > > devel@xxxxxxxxxxxxx; Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > > Subject: RE: vmx: VT-d posted-interrupt core logic handling
> > >
> > >
> > > Hi, Jan,
> > >
> > > I'm thinking your earlier idea about evenly distributed list:
> > >
> > > --
> > > Ah, right, I think that limitation was named before, yet I've
> > > forgotten about it again. But that only slightly alters the
> > > suggestion: To distribute vCPU-s evenly would then require to
> > > change their placement on the pCPU in the course of entering
> > > blocked state.
> > > --
> > >
> > > Actually after more thinking, there is no hard requirement that
> > > the vcpu must block on the pcpu which is configured in 'NDST'
> > > of that vcpu's PI descriptor. What really matters, is that the
> > > vcpu is added to the linked list of the very pcpu, then when PI
> > > notification comes we can always find out the vcpu struct from
> > > that pcpu's linked list. Of course one drawback of such placement
> > > is additional IPI incurred in wake up path.
> > >
> > > Then one possible optimized policy within vmx_vcpu_block could
> > > be:
> > >
> > > (Say PCPU1 which VCPU1 is currently blocked on)
> > > - As long as the #vcpus in the linked list on PCPU1 is below a
> > > threshold (say 16), add VCPU1 to the list. NDST set to PCPU1;
> > > Upon PI notification on PCPU1, local linked list is searched to
> > > find VCPU1 and then VCPU1 will be unblocked on PCPU1;
> > >
> > > - Otherwise, add VCPU1 to PCPU2 based on a simple distribution
> > > algorithm (based on vcpu_id/vm_id). VCPU1 still blocks on PCPU1
> > > but NDST set to PCPU2. Upon notification on PCPU2, local linked
> > > list is searched to find VCPU1 and then an IPI is sent to PCPU1 to
> > > unblock VCPU1;
> > >
> > > Feng, do you see any overlook here? :-)
> >
> > Kevin, thanks for the suggestion, it sounds a good idea, I will think
> > it a bit more and do some trials based on it.
> 
> Hey!
> 
> I am curious how the trials went? This feature is pretty awesome and
> I am wondering what the next step is?

Good to know that you are interested in this feature. However, I have
been occupied by another things recently, I think I will continue this
work some time later.

Thanks,
Feng

> 
> Thanks!

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