From: Jan Beulich <jbeulich@suse.com>
To: "Roger Pau Monné" <roger.pau@citrix.com>
Cc: Oleksandr Andrushchenko <Oleksandr_Andrushchenko@epam.com>,
"julien@xen.org" <julien@xen.org>,
"sstabellini@kernel.org" <sstabellini@kernel.org>,
Oleksandr Tyshchenko <Oleksandr_Tyshchenko@epam.com>,
Volodymyr Babchuk <Volodymyr_Babchuk@epam.com>,
Artem Mygaiev <Artem_Mygaiev@epam.com>,
"andrew.cooper3@citrix.com" <andrew.cooper3@citrix.com>,
"george.dunlap@citrix.com" <george.dunlap@citrix.com>,
"paul@xen.org" <paul@xen.org>,
Bertrand Marquis <bertrand.marquis@arm.com>,
Rahul Singh <rahul.singh@arm.com>,
"xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH v6 03/13] vpci: move lock outside of struct vpci
Date: Mon, 7 Feb 2022 17:12:57 +0100 [thread overview]
Message-ID: <ba35b9f5-158d-909f-5000-e789c8bfffa1@suse.com> (raw)
In-Reply-To: <YgFD8TFWwBqAe3U9@Air-de-Roger>
On 07.02.2022 17:08, Roger Pau Monné wrote:
> On Mon, Feb 07, 2022 at 04:26:56PM +0100, Jan Beulich wrote:
>> On 07.02.2022 16:11, Oleksandr Andrushchenko wrote:
>>>
>>>
>>> On 07.02.22 16:35, Oleksandr Andrushchenko wrote:
>>>>
>>>> On 07.02.22 16:27, Roger Pau Monné wrote:
>>>>> On Mon, Feb 07, 2022 at 03:11:03PM +0100, Jan Beulich wrote:
>>>>>> On 07.02.2022 14:53, Oleksandr Andrushchenko wrote:
>>>>>>> On 07.02.22 14:46, Roger Pau Monné wrote:
>>>>>>>> I think the per-domain rwlock seems like a good option. I would do
>>>>>>>> that as a pre-patch.
>>>>>>> It is. But it seems it won't solve the thing we started this adventure for:
>>>>>>>
>>>>>>> With per-domain read lock and still ABBA in modify_bars (hope the below
>>>>>>> is correctly seen with a monospace font):
>>>>>>>
>>>>>>> cpu0: vpci_write-> d->RLock -> pdev1->lock -> rom_write -> modify_bars: tmp (pdev2) ->lock
>>>>>>> cpu1: vpci_write-> d->RLock pdev2->lock -> cmd_write -> modify_bars: tmp (pdev1) ->lock
>>>>>>>
>>>>>>> There is no API to upgrade read lock to write lock in modify_bars which could help,
>>>>>>> so in both cases vpci_write should take write lock.
>>>>>> Hmm, yes, I think you're right: It's not modify_bars() itself which needs
>>>>>> to acquire the write lock, but its (perhaps indirect) caller. Effectively
>>>>>> vpci_write() would need to take the write lock if the range written
>>>>>> overlaps the BARs or the command register.
>>>>> I'm confused. If we use a per-domain rwlock approach there would be no
>>>>> need to lock tmp again in modify_bars, because we should hold the
>>>>> rwlock in write mode, so there's no ABBA?
>>>> this is only possible with what you wrote below:
>>>>> We will have however to drop the per domain read and vpci locks and
>>>>> pick the per-domain lock in write mode.
>>>> I think this is going to be unreliable. We need a reliable way to
>>>> upgrade read lock to write lock.
>>>> Then, we can drop pdev->vpci_lock at all, because we are always
>>>> protected with d->rwlock and those who want to free pdev->vpci
>>>> will use write lock.
>>>>
>>>> So, per-domain rwlock with write upgrade implemented minus pdev->vpci
>>>> should do the trick
>>> Linux doesn't implement write upgrade and it seems for a reason [1]:
>>> "Also, you cannot “upgrade” a read-lock to a write-lock, so if you at _any_ time
>>> need to do any changes (even if you don’t do it every time), you have to get
>>> the write-lock at the very beginning."
>>>
>>> So, I am not sure we can have the same for Xen...
>>>
>>> At the moment I see at least two possible ways to solve the issue:
>>> 1. Make vpci_write use write lock, thus make all write accesses synchronized
>>> for the given domain, read are fully parallel
>>
>> 1b. Make vpci_write use write lock for writes to command register and BARs
>> only; keep using the read lock for all other writes.
>
> We do not support writing to the BARs with memory decoding enabled
> currently for dom0, so we would only need to pick the lock in write
> mode for the command register and ROM BAR write handler AFAICT.
Oh, right - this then makes for even less contention due to needing to
acquire the lock in write mode.
Jan
next prev parent reply other threads:[~2022-02-07 16:13 UTC|newest]
Thread overview: 138+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-02-04 6:34 [PATCH v6 00/13] PCI devices passthrough on Arm, part 3 Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 01/13] xen/pci: arm: add stub for is_memory_hole Oleksandr Andrushchenko
2022-02-04 8:51 ` Julien Grall
2022-02-04 9:01 ` Oleksandr Andrushchenko
2022-02-04 9:41 ` Julien Grall
2022-02-04 9:47 ` Oleksandr Andrushchenko
2022-02-04 9:57 ` Julien Grall
2022-02-04 10:35 ` Oleksandr Andrushchenko
2022-02-04 11:00 ` Julien Grall
2022-02-04 11:25 ` Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 02/13] rangeset: add RANGESETF_no_print flag Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 03/13] vpci: move lock outside of struct vpci Oleksandr Andrushchenko
2022-02-04 7:52 ` Jan Beulich
2022-02-04 8:13 ` Oleksandr Andrushchenko
2022-02-04 8:36 ` Jan Beulich
2022-02-04 8:58 ` Oleksandr Andrushchenko
2022-02-04 9:15 ` Jan Beulich
2022-02-04 10:12 ` Oleksandr Andrushchenko
2022-02-04 10:49 ` Jan Beulich
2022-02-04 11:13 ` Roger Pau Monné
2022-02-04 11:37 ` Jan Beulich
2022-02-04 12:37 ` Oleksandr Andrushchenko
2022-02-04 12:47 ` Jan Beulich
2022-02-04 12:53 ` Oleksandr Andrushchenko
2022-02-04 13:03 ` Jan Beulich
2022-02-04 13:06 ` Roger Pau Monné
2022-02-04 14:43 ` Oleksandr Andrushchenko
2022-02-04 14:57 ` Roger Pau Monné
2022-02-07 11:08 ` Oleksandr Andrushchenko
2022-02-07 12:34 ` Jan Beulich
2022-02-07 12:57 ` Oleksandr Andrushchenko
2022-02-07 13:02 ` Jan Beulich
2022-02-07 12:46 ` Roger Pau Monné
2022-02-07 13:53 ` Oleksandr Andrushchenko
2022-02-07 14:11 ` Jan Beulich
2022-02-07 14:27 ` Roger Pau Monné
2022-02-07 14:33 ` Jan Beulich
2022-02-07 14:35 ` Oleksandr Andrushchenko
2022-02-07 15:11 ` Oleksandr Andrushchenko
2022-02-07 15:26 ` Jan Beulich
2022-02-07 16:07 ` Oleksandr Andrushchenko
2022-02-07 16:15 ` Jan Beulich
2022-02-07 16:21 ` Oleksandr Andrushchenko
2022-02-07 16:37 ` Jan Beulich
2022-02-07 16:44 ` Oleksandr Andrushchenko
2022-02-08 7:35 ` Oleksandr Andrushchenko
2022-02-08 8:57 ` Jan Beulich
2022-02-08 9:03 ` Oleksandr Andrushchenko
2022-02-08 10:50 ` Roger Pau Monné
2022-02-08 11:13 ` Oleksandr Andrushchenko
2022-02-08 13:38 ` Roger Pau Monné
2022-02-08 13:52 ` Oleksandr Andrushchenko
2022-02-08 8:53 ` Jan Beulich
2022-02-08 9:00 ` Oleksandr Andrushchenko
2022-02-08 10:11 ` Roger Pau Monné
2022-02-08 10:32 ` Oleksandr Andrushchenko
2022-02-07 16:08 ` Roger Pau Monné
2022-02-07 16:12 ` Jan Beulich [this message]
2022-02-07 14:28 ` Oleksandr Andrushchenko
2022-02-07 14:19 ` Roger Pau Monné
2022-02-07 14:27 ` Oleksandr Andrushchenko
2022-02-04 11:37 ` Oleksandr Andrushchenko
2022-02-04 12:15 ` Roger Pau Monné
2022-02-04 10:57 ` Roger Pau Monné
2022-02-04 6:34 ` [PATCH v6 04/13] vpci: restrict unhandled read/write operations for guests Oleksandr Andrushchenko
2022-02-04 14:11 ` Jan Beulich
2022-02-04 14:24 ` Oleksandr Andrushchenko
2022-02-08 8:00 ` Oleksandr Andrushchenko
2022-02-08 9:04 ` Jan Beulich
2022-02-08 9:09 ` Oleksandr Andrushchenko
2022-02-08 9:05 ` Roger Pau Monné
2022-02-08 9:10 ` Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 05/13] vpci: add hooks for PCI device assign/de-assign Oleksandr Andrushchenko
2022-02-07 16:28 ` Jan Beulich
2022-02-08 8:32 ` Oleksandr Andrushchenko
2022-02-08 9:13 ` Jan Beulich
2022-02-08 9:27 ` Oleksandr Andrushchenko
2022-02-08 9:44 ` Jan Beulich
2022-02-08 9:55 ` Oleksandr Andrushchenko
2022-02-08 10:09 ` Jan Beulich
2022-02-08 10:22 ` Oleksandr Andrushchenko
2022-02-08 10:29 ` Jan Beulich
2022-02-08 10:52 ` Oleksandr Andrushchenko
2022-02-08 11:00 ` Jan Beulich
2022-02-08 11:25 ` Oleksandr Andrushchenko
2022-02-10 8:21 ` Oleksandr Andrushchenko
2022-02-10 9:22 ` Jan Beulich
2022-02-10 9:33 ` Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 06/13] vpci/header: implement guest BAR register handlers Oleksandr Andrushchenko
2022-02-07 17:06 ` Jan Beulich
2022-02-08 8:06 ` Oleksandr Andrushchenko
2022-02-08 9:16 ` Jan Beulich
2022-02-08 9:29 ` Roger Pau Monné
2022-02-08 9:25 ` Roger Pau Monné
2022-02-08 9:31 ` Oleksandr Andrushchenko
2022-02-08 9:48 ` Jan Beulich
2022-02-08 9:57 ` Oleksandr Andrushchenko
2022-02-08 10:15 ` Jan Beulich
2022-02-08 10:29 ` Oleksandr Andrushchenko
2022-02-08 13:58 ` Roger Pau Monné
2022-02-04 6:34 ` [PATCH v6 07/13] vpci/header: handle p2m range sets per BAR Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 08/13] vpci/header: program p2m with guest BAR view Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 09/13] vpci/header: emulate PCI_COMMAND register for guests Oleksandr Andrushchenko
2022-02-04 14:25 ` Jan Beulich
2022-02-08 8:13 ` Oleksandr Andrushchenko
2022-02-08 9:33 ` Jan Beulich
2022-02-08 9:38 ` Oleksandr Andrushchenko
2022-02-08 9:52 ` Jan Beulich
2022-02-08 9:58 ` Oleksandr Andrushchenko
2022-02-08 11:11 ` Roger Pau Monné
2022-02-08 11:29 ` Oleksandr Andrushchenko
2022-02-08 14:09 ` Roger Pau Monné
2022-02-08 14:13 ` Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 10/13] vpci/header: reset the command register when adding devices Oleksandr Andrushchenko
2022-02-04 14:30 ` Jan Beulich
2022-02-04 14:37 ` Oleksandr Andrushchenko
2022-02-07 7:29 ` Jan Beulich
2022-02-07 11:27 ` Oleksandr Andrushchenko
2022-02-07 12:38 ` Jan Beulich
2022-02-07 12:51 ` Oleksandr Andrushchenko
2022-02-07 12:54 ` Jan Beulich
2022-02-07 14:17 ` Oleksandr Andrushchenko
2022-02-07 14:31 ` Jan Beulich
2022-02-07 14:46 ` Oleksandr Andrushchenko
2022-02-07 15:05 ` Jan Beulich
2022-02-07 15:14 ` Oleksandr Andrushchenko
2022-02-07 15:28 ` Jan Beulich
2022-02-07 15:59 ` Oleksandr Andrushchenko
2022-02-10 12:54 ` Oleksandr Andrushchenko
2022-02-10 13:36 ` Jan Beulich
2022-02-10 13:56 ` Oleksandr Andrushchenko
2022-02-10 12:59 ` Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 11/13] vpci: add initial support for virtual PCI bus topology Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 12/13] xen/arm: translate virtual PCI bus topology for guests Oleksandr Andrushchenko
2022-02-04 7:56 ` Jan Beulich
2022-02-04 8:18 ` Oleksandr Andrushchenko
2022-02-04 6:34 ` [PATCH v6 13/13] xen/arm: account IO handlers for emulated PCI MSI-X Oleksandr Andrushchenko
2022-02-11 15:28 ` Julien Grall
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