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From: Peter Xu <peterx@redhat.com>
To: "Maciej S. Szmigiero" <mail@maciej.szmigiero.name>
Cc: "Dr. David Alan Gilbert" <dave@treblig.org>,
	"Fabiano Rosas" <farosas@suse.de>,
	"Alex Williamson" <alex.williamson@redhat.com>,
	"Cédric Le Goater" <clg@redhat.com>,
	"Eric Blake" <eblake@redhat.com>,
	"Markus Armbruster" <armbru@redhat.com>,
	"Daniel P. Berrangé" <berrange@redhat.com>,
	"Avihai Horon" <avihaih@nvidia.com>,
	"Joao Martins" <joao.m.martins@oracle.com>,
	qemu-devel@nongnu.org
Subject: Re: [PATCH v4 09/33] migration: postcopy_ram_listen_thread() needs to take BQL for some calls
Date: Tue, 4 Feb 2025 10:39:51 -0500	[thread overview]
Message-ID: <Z6I0x0hVc-6f1-v4@x1.local> (raw)
In-Reply-To: <27e3cb26-43e0-4505-84da-865c79af680a@maciej.szmigiero.name>

On Tue, Feb 04, 2025 at 03:57:37PM +0100, Maciej S. Szmigiero wrote:
> The vfio_migration_cleanup() used to just close a migration FD, while
> RAM might end up calling qemu_ram_msync(), which sounds like something
> that should be called under BQL.
> 
> But I am not sure whether that lack of BQL around qemu_ram_msync()
> actually causes problems.

I believe msync() is thread-safe.  So that doesn't need BQL, AFAICT.

Personally I actually prefer not having the BQL requirement if ever
possible in any vmstate hooks.

I think the only challenge here is if VFIO will start to need BQL for some
specific code path that you added in this series, it means VFIO needs to
detect bql_locked() to make sure it won't deadlock.. and only take BQL if
it's not taken.

From that POV, it might be easier for you to define that hook as "always do
cleanup() with BQL" globally, just to avoid one bql_locked() usage in vfio
specific hook.  We pay that with slow RAM sync in corner cases like pmem
that could potentially block VM from making progress (e.g. vcpu
concurrently accessing MMIO regions).

Thanks,

-- 
Peter Xu



  reply	other threads:[~2025-02-04 15:40 UTC|newest]

Thread overview: 137+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2025-01-30 10:08 [PATCH v4 00/33] Multifd 🔀 device state transfer support with VFIO consumer Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 01/33] migration: Clarify that {load, save}_cleanup handlers can run without setup Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 02/33] thread-pool: Remove thread_pool_submit() function Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 03/33] thread-pool: Rename AIO pool functions to *_aio() and data types to *Aio Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 04/33] thread-pool: Implement generic (non-AIO) pool support Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 05/33] migration: Add MIG_CMD_SWITCHOVER_START and its load handler Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 06/33] migration: Add qemu_loadvm_load_state_buffer() and its handler Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 07/33] io: tls: Allow terminating the TLS session gracefully with EOF Maciej S. Szmigiero
2025-02-04 15:15   ` Daniel P. Berrangé
2025-02-04 16:02     ` Maciej S. Szmigiero
2025-02-04 16:14       ` Daniel P. Berrangé
2025-02-04 18:25         ` Maciej S. Szmigiero
2025-02-06 21:53           ` Peter Xu
2025-01-30 10:08 ` [PATCH v4 08/33] migration/multifd: Allow premature EOF on TLS incoming channels Maciej S. Szmigiero
2025-02-03 18:20   ` Peter Xu
2025-02-03 18:53     ` Maciej S. Szmigiero
2025-02-03 20:20       ` Peter Xu
2025-02-03 21:41         ` Maciej S. Szmigiero
2025-02-03 22:56           ` Peter Xu
2025-02-04 13:51             ` Fabiano Rosas
2025-02-04 14:39             ` Maciej S. Szmigiero
2025-02-04 15:00               ` Fabiano Rosas
2025-02-04 15:10                 ` Maciej S. Szmigiero
2025-02-04 15:31               ` Peter Xu
2025-02-04 15:39                 ` Daniel P. Berrangé
2025-02-05 19:09                   ` Fabiano Rosas
2025-02-05 20:42                     ` Fabiano Rosas
2025-02-05 20:55                       ` Maciej S. Szmigiero
2025-02-06 14:13                         ` Fabiano Rosas
2025-02-06 14:53                           ` Maciej S. Szmigiero
2025-02-06 15:20                             ` Fabiano Rosas
2025-02-06 16:01                               ` Maciej S. Szmigiero
2025-02-06 17:32                                 ` Fabiano Rosas
2025-02-06 17:55                                   ` Maciej S. Szmigiero
2025-02-06 21:51                                   ` Peter Xu
2025-02-07 13:17                                     ` Fabiano Rosas
2025-02-07 14:04                                       ` Peter Xu
2025-02-07 14:16                                         ` Fabiano Rosas
2025-02-05 21:13                       ` Peter Xu
2025-02-06 14:19                         ` Fabiano Rosas
2025-02-04 15:10         ` Daniel P. Berrangé
2025-02-04 15:08     ` Daniel P. Berrangé
2025-02-04 16:02       ` Peter Xu
2025-02-04 16:12         ` Daniel P. Berrangé
2025-02-04 16:29           ` Peter Xu
2025-02-04 18:25         ` Fabiano Rosas
2025-02-04 19:34           ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 09/33] migration: postcopy_ram_listen_thread() needs to take BQL for some calls Maciej S. Szmigiero
2025-02-02  2:06   ` Dr. David Alan Gilbert
2025-02-02 11:55     ` Maciej S. Szmigiero
2025-02-02 12:45       ` Dr. David Alan Gilbert
2025-02-03 13:57         ` Maciej S. Szmigiero
2025-02-03 19:58           ` Peter Xu
2025-02-03 20:15             ` Maciej S. Szmigiero
2025-02-03 20:36               ` Peter Xu
2025-02-03 21:41                 ` Maciej S. Szmigiero
2025-02-03 23:02                   ` Peter Xu
2025-02-04 14:57                     ` Maciej S. Szmigiero
2025-02-04 15:39                       ` Peter Xu [this message]
2025-02-04 19:32                         ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 10/33] error: define g_autoptr() cleanup function for the Error type Maciej S. Szmigiero
2025-02-03 20:53   ` Peter Xu
2025-02-03 21:13   ` Daniel P. Berrangé
2025-02-03 21:51     ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 11/33] migration: Add thread pool of optional load threads Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 12/33] migration/multifd: Split packet into header and RAM data Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 13/33] migration/multifd: Device state transfer support - receive side Maciej S. Szmigiero
2025-02-03 21:27   ` Peter Xu
2025-02-03 22:18     ` Maciej S. Szmigiero
2025-02-03 22:59       ` Peter Xu
2025-02-04 14:40         ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 14/33] migration/multifd: Make multifd_send() thread safe Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 15/33] migration/multifd: Add an explicit MultiFDSendData destructor Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 16/33] migration/multifd: Device state transfer support - send side Maciej S. Szmigiero
2025-02-03 21:47   ` Peter Xu
2025-01-30 10:08 ` [PATCH v4 17/33] migration/multifd: Make MultiFDSendData a struct Maciej S. Szmigiero
2025-02-07 14:36   ` Fabiano Rosas
2025-02-07 19:43     ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 18/33] migration/multifd: Add multifd_device_state_supported() Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 19/33] migration: Add save_live_complete_precopy_thread handler Maciej S. Szmigiero
2025-02-04 17:54   ` Peter Xu
2025-02-04 19:32     ` Maciej S. Szmigiero
2025-02-04 20:34       ` Peter Xu
2025-02-05 11:53         ` Maciej S. Szmigiero
2025-02-05 15:55           ` Peter Xu
2025-02-06 11:41             ` Maciej S. Szmigiero
2025-02-06 22:16               ` Peter Xu
2025-01-30 10:08 ` [PATCH v4 20/33] vfio/migration: Add x-migration-load-config-after-iter VFIO property Maciej S. Szmigiero
2025-02-10 17:24   ` Cédric Le Goater
2025-02-11 14:37     ` Maciej S. Szmigiero
2025-02-11 15:00       ` Cédric Le Goater
2025-02-11 15:57         ` Maciej S. Szmigiero
2025-02-11 16:28           ` Cédric Le Goater
2025-01-30 10:08 ` [PATCH v4 21/33] vfio/migration: Add load_device_config_state_start trace event Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 22/33] vfio/migration: Convert bytes_transferred counter to atomic Maciej S. Szmigiero
2025-01-30 21:35   ` Cédric Le Goater
2025-01-31  9:47     ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 23/33] vfio/migration: Multifd device state transfer support - basic types Maciej S. Szmigiero
2025-02-10 17:17   ` Cédric Le Goater
2025-01-30 10:08 ` [PATCH v4 24/33] vfio/migration: Multifd device state transfer support - VFIOStateBuffer(s) Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 25/33] vfio/migration: Multifd device state transfer - add support checking function Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 26/33] vfio/migration: Multifd device state transfer support - receive init/cleanup Maciej S. Szmigiero
2025-02-12 10:55   ` Cédric Le Goater
2025-02-14 20:55     ` Maciej S. Szmigiero
2025-02-17  9:38       ` Cédric Le Goater
2025-02-17 22:13         ` Maciej S. Szmigiero
2025-02-18  7:54           ` Cédric Le Goater
2025-01-30 10:08 ` [PATCH v4 27/33] vfio/migration: Multifd device state transfer support - received buffers queuing Maciej S. Szmigiero
2025-02-12 13:47   ` Cédric Le Goater
2025-02-14 20:58     ` Maciej S. Szmigiero
2025-02-17 13:48       ` Cédric Le Goater
2025-02-17 22:15         ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 28/33] vfio/migration: Multifd device state transfer support - load thread Maciej S. Szmigiero
2025-02-12 15:48   ` Cédric Le Goater
2025-02-12 16:19     ` Cédric Le Goater
2025-02-17 22:09       ` Maciej S. Szmigiero
2025-02-17 22:09     ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 29/33] vfio/migration: Multifd device state transfer support - config loading support Maciej S. Szmigiero
2025-02-12 16:21   ` Cédric Le Goater
2025-02-17 22:09     ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 30/33] migration/qemu-file: Define g_autoptr() cleanup function for QEMUFile Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 31/33] vfio/migration: Multifd device state transfer support - send side Maciej S. Szmigiero
2025-02-12 17:03   ` Cédric Le Goater
2025-02-17 22:12     ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 32/33] vfio/migration: Add x-migration-multifd-transfer VFIO property Maciej S. Szmigiero
2025-02-12 17:10   ` Cédric Le Goater
2025-02-14 20:56     ` Maciej S. Szmigiero
2025-02-17 13:57       ` Cédric Le Goater
2025-02-17 14:16         ` Maciej S. Szmigiero
2025-01-30 10:08 ` [PATCH v4 33/33] hw/core/machine: Add compat for " Maciej S. Szmigiero
2025-01-30 20:19 ` [PATCH v4 00/33] Multifd 🔀 device state transfer support with VFIO consumer Fabiano Rosas
2025-01-30 20:27   ` Maciej S. Szmigiero
2025-01-30 20:46     ` Fabiano Rosas
2025-01-31 18:16     ` Maciej S. Szmigiero
2025-02-03 14:19 ` Cédric Le Goater
2025-02-21  6:57   ` Yanghang Liu
2025-02-22  9:51     ` Maciej S. Szmigiero

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