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

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

> -----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.

Thanks,
Feng

> 
> Thanks
> Kevin

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