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

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

> -----Original Message-----
> From: Dario Faggioli [mailto:dario.faggioli@xxxxxxxxxx]
> Sent: Monday, September 21, 2015 9:32 PM
> To: Wu, Feng; George Dunlap
> Cc: xen-devel@xxxxxxxxxxxxx; Tian, Kevin; Keir Fraser; George Dunlap; Andrew
> Cooper; Jan Beulich
> Subject: Re: [Xen-devel] [PATCH v7 15/17] vmx: VT-d posted-interrupt core 
> logic
> handling
> 
> On Mon, 2015-09-21 at 11:59 +0000, Wu, Feng wrote:
> >
> > > -----Original Message-----
> > > From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> 
> > > > I think the handling for lazy context switch is not only for the
> > > > blocking case,
> > > > we still need to do something for lazy context switch even we
> > > > handled the
> > > > blocking case in vcpu_block(), such as,
> > > > 1. For non-idle -> idle
> > > > - set 'SN'
> > >
> > > If we set SN in vcpu_block(), then we don't need to set it on
> > > context
> > > switch -- æäæï
> >
> > For preemption case (not blocking case) , we still need to clear/set
> > SN, and
> > this has no business with vcpu_block()/vcpu_wake(), right? Do I miss
> > something
> > here? BTW, you Chinese is good! :)
> >
> Well, sure, preemptions are fine being dealt with during context
> switches.
> 
> AFAICR, Geoge was suggesting investigating the possibility of doing
> that within the already existing arch specific part of the context
> switch itself. Have you checked whether that would be possible? If yes,
> it really would be great, IMO.

This is George's suggestion about this:

" And at that point, could we actually get rid of the PI-specific context
switch hooks altogether, and just put the SN state changes required for
running->runnable and runnable->running in vmx_ctxt_switch_from() and
vmx_ctxt_switch_to()?"

However, vmx_ctxt_switch_from() and vmx_ctxt_switch_to() are called in
__context_switch(), which still cannot cover "non-idle -> idle" and "idle -> 
non-idle"
lazy transition. And I think we need to change SN's state during the two
transitions.

> 
> Note that, in case of preemptions, we are switching from a non-idle
> vcpu to another, non-idle, vcpu, so lazy context switching to the idle
> vcpu should not have much to do with this case... 

So do you mean in preemptions, we cannot switch from non-idle to idle or
Idle to non-idle, i.e, we can only switch from non-idle to non-idle?

Thanks,
Feng

> Was this something
> you were saying something/asking about above? (seems so, but I can't be
> sure, so I thought I better ask :-) ).



> 
> 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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