Re: [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list

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

> -----Original Message-----
> From: Dario Faggioli [mailto:dario.faggioli@citrix.com]
> Sent: Thursday, June 23, 2016 5:33 AM
> To: Wu, Feng <feng.wu@intel.com>; xen-devel@lists.xen.org
> Cc: keir@xen.org; Tian, Kevin <kevin.tian@intel.com>; jbeulich@suse.com;
> andrew.cooper3@citrix.com; george.dunlap@eu.citrix.com;
> konrad.wilk@oracle.com
> Subject: Re: [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu
> blocking list
> 
> On Tue, 2016-05-31 at 10:19 +0000, Wu, Feng wrote:
> > > -----Original Message-----
> > > From: Dario Faggioli [mailto:dario.faggioli@citrix.com]
> > >
> > > So, if you want try argumenting a bit on what was your line of
> > > reasoning when doing things this way, that would be helpful (at
> > > least
> > > to me).
> > 'pi_hotplug_lock' is trying to protect the following scenario:
> > vmx_pi_blocking_cleanup() gets called either when the last assigned
> > device is detached or the vCPU is going to be destroyed, and at the
> > same time vmx_vcpu_block() is running. We need to make sure
> > after all the blocking vCPU is cleaned up, we should not add new
> > vCPU to the per-cpu blocking list. And that is why I introduce
> > ' pi_blocking_cleaned_up' for each vCPU, which being set to
> > 1 means that we cannot add it to the blocking list in
> > vmx_vcpu_block().
> >
> By "the vCPU is going to be destroyed", to what code path do you refer?
> Because, for instance, there's this:
> 
>   domain_kill() --> domain_destroy() --> complete_domain_destroy() --
>   --> vcpu_destroy() --> hvm_vcpu_destroy()

Yes, this is the case I was thinking of.

> 
> in which case, the vCPUs are not running --and hence can't block--
> during their own destruction.

Thanks for the clarification. That is good I don't need to consider
this case.:)

> 
> I take that you've found a path where that does not hold, and hence
> requires this kind of protection?
> 
> For the other race (last device being unassigned), I'll look more into
> it, but, in general, I share George's and Jan's views that we need
> simpler, more consistent and easier to maintain solutions.

Thanks for your time and looking forward to your comments!

> 
> > For the other flag 'down', it is used for the following scenario:
> > When a pCPU is going away and meanwhile vmx_vcpu_block() is
> > called, we should not put the vCPU to a per-cpu blocking list, which
> > is going away.
> >
> But, in this case, as George basically said already, if a pCPU is being
> destroyed, there should be no vCPU running on it, and hence no vCPU
> that, if blocking, would need being added to the pCPU blocking list.
> 
> > > For instance, now arch_vcpu_block() returns a value and, as you say
> > > yourself in a comment, that is for (potentially) preventing a vcpu
> > > to
> > > block. So the behavior of schedule.c:vcpu_block(), now depends on
> > > your
> > > new flag per_cpu(vmx_pi_blocking, v->processor).down. Again, I'll
> > > look
> > > better, but this has few chances of being right (at least
> > > logically).
> > Like in vcpu_block(),it will check events before actually blocking
> > the vcpu,
> > here we just introduce another case in which the vCPU cannot be
> > blocked.
> > I don't know why you think this is problematic?
> >
> Well, but, right now, it's like this:
>  - the vCPU should block, waiting for an event
>  - it turns out the event is already arrive
>  - we can avoid blocking
> 
> In your case, AFAICUI, it's:
>  - the vCPU should block, waiting for an event
>  - the event is _not_ arrived, so we indeed should block
>  - we do *not* block, due to some other reason
> 
> That does not look right to me... what about the fact that we wanted to
> actually wait for the event? :-O

I understand your point. In my understanding, currently, vcpu_block() is
for guest's HLT operation, which means, guest has nothing to do. In
this case, even we return (not blocking), seems the function is not
broken.

Thanks,
Feng

> 
> 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)

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
http://lists.xen.org/xen-devel