Re: [Xen-devel] [PATCH 0/3] VMX: Properly handle pi descriptor and per-cpu blocking list

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

> -----Original Message-----
> From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
> Sent: Friday, May 20, 2016 6:27 PM
> To: Wu, Feng <feng.wu@xxxxxxxxx>
> Cc: andrew.cooper3@xxxxxxxxxx; dario.faggioli@xxxxxxxxxx;
> george.dunlap@xxxxxxxxxxxxx; Tian, Kevin <kevin.tian@xxxxxxxxx>; xen-
> devel@xxxxxxxxxxxxx; konrad.wilk@xxxxxxxxxx; keir@xxxxxxx
> Subject: Re: [PATCH 0/3] VMX: Properly handle pi descriptor and per-cpu
> blocking list
> 
> >>> On 20.05.16 at 10:53, <feng.wu@xxxxxxxxx> wrote:
> > I still have two opens, which needs comments/suggestions from you guys.
> > - What should we do for the per-cpu blocking list during vcpu hotplug?
> 
> What do you mean with vcpu hotplug? vcpus never get removed
> from a VM (from hypervisor perspective), and all the guest may
> ever use need to be created before the guest starts.

Thanks for the reply, Jan. First of all, I am not familiar with vcpu 
hotplug/pcup hotplug,
and that is why I list them as two opens here. When I wrote "vcpu hotplug", I 
was planning
to refer to the following case:
1. use 'xl vcpu-set ${dom_id} ${vcpu_num}', where ${vcpu_num} is less than the 
current
online vcpus.
2. In the guest, use ' echo 1 > /sys/devices/system/cpu/cpuX/online ' to make 
the vcpus
offline.

Yes, I know the maximum vcpus are allocated for the guest, but I am not quite 
sure
whether the above operations will affect the blocking list. If you can 
elaborate a bit
more what hypervisor will do for the above operations, That should be very 
helpful.
such as, will the vcpu's state be changed, etc? And if the vcpu is current in 
blocking
state, while try to offline it, it will still remain in the blocking list, 
right, will this cause
some problems? 

> 
> > - What should we do for the per-cpu blocking list during pcpu hotplug?
> 
> I think it would help if you said what issue you see.

I didn't see any issues related to this, this open just comes out in my mind 
when I wrote
this patch to fix the bug. As mentioned above, when the pCPU is removed, what 
is the
status of its blocking list? But thinking a bit more about this, I feel it 
should work this way,
before the pCPU is removed, all the vCPUs running on it has been moved to 
another
pCPU, right? If this is the case, it can address part of my concern. Another 
concern is
if a vCPU is blocking on a pCPU, then the pCPU is going to be unplugged, what 
does
Xen do for the blocking vCPU?

Thanks,
Feng

> 
> Jan


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