Re: [Xen-devel] [v3 12/15] vmx: posted-interrupt handling when vCPU is blocked

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

> -----Original Message-----
> From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]
> Sent: Thursday, July 02, 2015 8:04 PM
> To: Andrew Cooper
> Cc: Wu, Feng; Tian, Kevin; keir@xxxxxxx; george.dunlap@xxxxxxxxxxxxx;
> xen-devel@xxxxxxxxxxxxx; jbeulich@xxxxxxxx; Zhang, Yang Z
> Subject: Re: [Xen-devel] [v3 12/15] vmx: posted-interrupt handling when vCPU
> is blocked
> 
> On Thu, 2015-07-02 at 11:30 +0100, Andrew Cooper wrote:
> > On 02/07/15 09:30, Dario Faggioli wrote:
> 
> > > It is, therefore, not effective in making sure that, even with only one
> > > notification, you only kick the interested vcpu.
> > >
> > > This is the third time that I ask:
> > >  (1) whether it is possible to have more vcpus queued on one pcpu PI
> > >      blocked list with desc.on (I really believe it is);
> > >  (2) if yes, whether it is TheRightThing(TM) to kick all of them, as
> > >      soon as any notification arrives, instead that putting together a
> > >      mechanism for kicking only a specific one.
> >
> > We will receive one NV for every time the hardware managed to
> > successfully set desc.on
> >
> Right, I see it now, thanks.
> 
> > If multiple stack up and we proactively drain the list, we will
> > subsequently search the list to completion for all remaining NV's, due
> > to finding no appropriate entries.
> >
> > I can't currently decide whether this will be quicker or slower overall,
> > or (most likely) it will even out to equal in the general case.
> >
> Well, given the thing works as you (two) just described, I think
> draining the list is the only thing we can do.
> 
> In fact, AFAICT, since we can't know for what vcpu a particular
> notification is intended,

Exactly, when notification event happens, the hardware sets 'ON',
software will find the vCPU with 'ON' set, in fact, software doesn't
know which vCPU the wakeup event is targeting, the only thing it
can do is kicking the vCPUs with desc.on = 1.

Thanks,
Feng

 we don't have alternatives to waking them all,
> do we?
> 
> 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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