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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next prev parent 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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