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From: "Jan Beulich" <JBeulich@suse.com>
To: <andrii.anisov@gmail.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Wei Liu <wei.liu2@citrix.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>,
	George Dunlap <George.Dunlap@eu.citrix.com>,
	Andrew Cooper <andrew.cooper3@citrix.com>,
	Ian Jackson <Ian.Jackson@eu.citrix.com>, Tim Deegan <tim@xen.org>,
	Julien Grall <julien.grall@arm.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	"andrii_anisov@epam.com" <andrii_anisov@epam.com>,
	Roger Pau Monne <roger.pau@citrix.com>
Subject: Re: [Xen-devel] [PATCH v3] xen: introduce VCPUOP_register_runstate_phys_memory_area hypercall
Date: Thu, 13 Jun 2019 06:41:50 -0600	[thread overview]
Message-ID: <5D02448E0200007800237D77@prv1-mh.provo.novell.com> (raw)
In-Reply-To: <7de133a9-6aa7-1523-3f8f-b5506efef032@gmail.com>

>>> On 13.06.19 at 14:32, <andrii.anisov@gmail.com> wrote:
> Jan, Julien,
> 
> On 11.06.19 12:10, Jan Beulich wrote:
>>>> At the very least such loops want a cpu_relax() in their bodies.
>>>> But this being on a hypercall path - are there theoretical guarantees
>>>> that a guest can't abuse this to lock up a CPU?
>>> Hmmm, I suggested this but it looks like a guest may call the hypercall 
> multiple
>>> time from different vCPU. So this could be a way to delay work on the CPU.
>>>
>>> I wanted to make the context switch mostly lockless and therefore avoiding 
> to
>>> introduce a spinlock.
>> 
>> Well, constructs like the above are trying to mimic a spinlock
>> without actually using a spinlock. There are extremely rare
>> situation in which this may indeed be warranted, but here it
>> falls in the common "makes things worse overall" bucket, I
>> think. To not unduly penalize the actual update paths, I think
>> using a r/w lock would be appropriate here.
> 
> So what is the conclusion here? Should we go with trylock and 
> hypercall_create_continuation() in order to avoid locking but still not fail 
> to the guest?

I'm not convinced a "trylock" approach is needed - that's
something Julien suggested. I'm pretty sure we're acquiring other
locks in hypercall context without going the trylock route. I am
convinced though that the pseudo-lock you've used needs to be
replaced by a real (and perhaps r/w) one, _if_ there is any need
for locking in the first place.

Jan



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  reply	other threads:[~2019-06-13 12:42 UTC|newest]

Thread overview: 51+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-24 18:12 [PATCH RFC 2] [DO NOT APPLY] introduce VCPUOP_register_runstate_phys_memory_area hypercall Andrii Anisov
2019-05-24 18:12 ` [Xen-devel] " Andrii Anisov
2019-05-24 18:12 ` [PATCH v3] Introduce runstate area registration with phys address Andrii Anisov
2019-05-24 18:12   ` [Xen-devel] " Andrii Anisov
2019-05-24 18:12 ` [PATCH v3] xen: introduce VCPUOP_register_runstate_phys_memory_area hypercall Andrii Anisov
2019-05-24 18:12   ` [Xen-devel] " Andrii Anisov
2019-06-07 14:23   ` Jan Beulich
2019-06-10 11:44     ` Julien Grall
2019-06-11  9:10       ` Jan Beulich
2019-06-11 10:22         ` Andrii Anisov
2019-06-11 12:12           ` Julien Grall
2019-06-11 12:26             ` Andrii Anisov
2019-06-11 12:32               ` Julien Grall
2019-06-11 12:40                 ` Andrii Anisov
2019-06-13 12:21           ` Andrii Anisov
2019-06-13 12:39             ` Jan Beulich
2019-06-13 12:32         ` Andrii Anisov
2019-06-13 12:41           ` Jan Beulich [this message]
2019-06-13 12:48             ` Julien Grall
2019-06-13 12:58               ` Jan Beulich
2019-06-13 13:14                 ` Julien Grall
2019-06-13 13:40                   ` Jan Beulich
2019-06-13 14:41                     ` Julien Grall
2019-06-14 14:36                       ` Andrii Anisov
2019-06-14 14:39                         ` Julien Grall
2019-06-14 15:11                           ` Andrii Anisov
2019-06-14 15:24                             ` Julien Grall
2019-06-14 16:11                               ` Andrii Anisov
2019-06-14 16:20                                 ` Julien Grall
2019-06-14 16:25                                   ` Andrii Anisov
2019-06-17  6:27                                     ` Jan Beulich
2019-06-14 15:42                             ` Jan Beulich
2019-06-14 16:23                               ` Andrii Anisov
2019-06-17  6:28                                 ` Jan Beulich
2019-06-18 15:32                                   ` Andrii Anisov
2019-06-18 15:44                                     ` Jan Beulich
2019-06-11 16:09     ` Andrii Anisov
2019-06-12  7:27       ` Jan Beulich
2019-06-13 12:17         ` Andrii Anisov
2019-06-13 12:36           ` Jan Beulich
2019-06-11 16:13     ` Andrii Anisov
2019-05-24 18:12 ` [PATCH RFC 1] [DO NOT APPLY] " Andrii Anisov
2019-05-24 18:12   ` [Xen-devel] " Andrii Anisov
2019-05-28  8:59 ` [PATCH RFC 2] " Julien Grall
2019-05-28  8:59   ` [Xen-devel] " Julien Grall
2019-05-28  9:17   ` Andrii Anisov
2019-05-28  9:17     ` [Xen-devel] " Andrii Anisov
2019-05-28  9:23     ` Julien Grall
2019-05-28  9:23       ` [Xen-devel] " Julien Grall
2019-05-28  9:36       ` Andrii Anisov
2019-05-28  9:36         ` [Xen-devel] " Andrii Anisov

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