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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 20.05.16 at 12:46, <feng.wu@xxxxxxxxx> wrote:

> 
>> -----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.

The hypervisor just offlines the vCPU (in response to the guest
invoking VCPUOP_down).

> 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? 

Since _VPF_down is just one of many possible pause flags, I don't
see what problems you're suspecting. But maybe I'm overlooking
something?

>> > - 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?

Yes.

> 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?

A pCPU is never blocking "on a pCPU", it is always just blocking.
vCPU-s currently having their v->processor set to the pCPU being
hot removed would simply get migrated elsewhere. If that's not
accompanied by respective PI blocking list adjustments, then I can
only refer you back to the original series' review process, where
it was (iirc) more than once that it was pointed out that getting out
of sync v->processor and the PI list a vCPU sits on may casue
issues. And if there is an issue here, I'm not sure what a good
solution would be.

Jan


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