From: Dario Faggioli <dario.faggioli@citrix.com>
To: "Wu, Feng" <feng.wu@intel.com>,
"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
Cc: "Tian, Kevin" <kevin.tian@intel.com>,
"keir@xen.org" <keir@xen.org>,
"george.dunlap@eu.citrix.com" <george.dunlap@eu.citrix.com>,
"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
"jbeulich@suse.com" <jbeulich@suse.com>
Subject: Re: [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list
Date: Wed, 22 Jun 2016 23:33:07 +0200 [thread overview]
Message-ID: <1466631187.18398.55.camel@citrix.com> (raw)
In-Reply-To: <E959C4978C3B6342920538CF579893F0197009A6@SHSMSX103.ccr.corp.intel.com>
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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()
in which case, the vCPUs are not running --and hence can't block--
during their own destruction.
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.
> 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
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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next prev parent reply other threads:[~2016-06-22 21:33 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-26 13:39 [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list Feng Wu
2016-05-26 13:39 ` [PATCH v2 1/4] VMX: Properly handle pi when all the assigned devices are removed Feng Wu
2016-05-27 13:43 ` Jan Beulich
2016-05-31 10:22 ` Wu, Feng
2016-05-31 11:52 ` Jan Beulich
2016-06-03 5:12 ` Wu, Feng
2016-06-03 9:45 ` Jan Beulich
2016-05-26 13:39 ` [PATCH v2 2/4] VMX: Cleanup PI per-cpu blocking list when vcpu is destroyed Feng Wu
2016-05-27 13:49 ` Jan Beulich
2016-05-31 10:22 ` Wu, Feng
2016-05-31 11:54 ` Jan Beulich
2016-05-26 13:39 ` [PATCH v2 3/4] VMX: Assign the right value to 'NDST' field in a concern case Feng Wu
2016-05-27 14:00 ` Jan Beulich
2016-05-31 10:27 ` Wu, Feng
2016-05-31 11:58 ` Jan Beulich
2016-06-03 5:23 ` Wu, Feng
2016-06-03 9:57 ` Jan Beulich
2016-06-22 18:00 ` George Dunlap
2016-06-24 9:08 ` Wu, Feng
2016-05-26 13:39 ` [PATCH v2 4/4] VMX: fixup PI descritpor when cpu is offline Feng Wu
2016-05-27 14:56 ` Jan Beulich
2016-05-31 10:31 ` Wu, Feng
2016-06-22 18:33 ` George Dunlap
2016-06-24 6:34 ` Wu, Feng
2016-05-26 17:20 ` [PATCH v2 0/4] VMX: Properly handle pi descriptor and per-cpu blocking list Dario Faggioli
2016-05-31 10:19 ` Wu, Feng
2016-06-22 21:33 ` Dario Faggioli [this message]
2016-06-24 6:33 ` Wu, Feng
2016-06-24 10:29 ` Dario Faggioli
2016-06-24 13:42 ` Wu, Feng
2016-06-24 13:49 ` George Dunlap
2016-06-24 14:36 ` Dario Faggioli
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