Re: [PULL v4 76/83] vhost-user: Support vhost_dev_start

[Dr. David Alan Gilbert]

* Greg Kurz (groug@kaod.org) wrote:
> On Tue, 17 Jan 2023 13:12:57 +0100
> Greg Kurz <groug@kaod.org> wrote:
> 
> > Hi Maxime,
> > 
> > On Tue, 17 Jan 2023 10:49:37 +0100
> > Maxime Coquelin <maxime.coquelin@redhat.com> wrote:
> > 
> > > Hi Yajun,
> > > 
> > > On 1/16/23 08:14, Yajun Wu wrote:
> > > > Not quite sure about the whole picture.
> > > > 
> > > > Seems while qemu waiting response of vhost_user_get_status, dpdk send 
> > > > out VHOST_USER_SLAVE_IOTLB_MSG and trigger qemu function 
> > > > vhost_backend_update_device_iotlb.
> > > > Qemu wait on reply of VHOST_USER_IOTLB_MSG but get 
> > > > VHOST_USER_GET_STATUS reply.
> > > 
> > > Thanks for the backtrace, that helps a lot.
> > > 
> > > The issue happens because:
> > >   1. Introduction of nested event loop in vhost_user_read() [0] features
> > > that enables handling slave channel request while waiting for reply on
> > > the masyer channel.
> > >   2. Slave VHOST_USER_SLAVE_IOTLB_MSG slave request handling ends-up 
> > > sending a VHOST_USER_IOTLB_MSG on the master channel.
> > > 
> > > So while waiting for VHOST_USER_IOTLB_MSG reply, it receives the reply
> > > for the first request sent on the master channel, here the
> > > VHOST_USER_GET_STATUS reply.
> > > 
> > > I don't see an easy way to fix it.
> > > 
> > > One option would be to have the slave channel being handled by another
> > > thread, and protect master channel with a lock to enforce the
> > > synchronization. But this may induce other issues, so that's not a light
> > > change.
> > > 
> > 
> > This is going to be tough because the back-end might have set the
> > VHOST_USER_NEED_REPLY_MASK flag on the VHOST_USER_SLAVE_IOTLB_MSG
> > request and thus might be waiting for a response on the slave
> > channel. In order to emit such a response, the front-end must
> > send VHOST_USER_SLAVE_IOTLB_MSG updates on the master channel *first*
> > according to the protocol specification. This means that we really
> > cannot handle VHOST_USER_SLAVE_IOTLB_MSG requests while there's
> > an on-going transaction on the master channel.
> > 
> > > (Adding Greg and Stefan, who worked on the nested event loop series.)
> > > 
> > > Simply reverting nested event loop support may not be an option, since
> > > it would break virtiofsd, as if my understanding is correct, waits for
> > > some slave channel request to complete in order to complete a request
> > > made by QEMU on the master channel.
> > > 
> > > Any thougths?
> > > 
> > 
> > Well... the nested even loop was added as preparatory work for "the
> > upcoming enablement of DAX with virtio-fs". This requires changes on
> > the QEMU side that haven't been merged yet. Technically, it seems that
> > reverting the nested event loop won't break anything in upstream QEMU
> > at this point (but this will bite again as soon as DAX enablement gets
> > merged).
> > 
> 
> Cc'ing Dave to know about the DAX enablement status.

Yeh not too much to say; but having just sent the patches to nuke the C
virtiofsd, the plan is to look at the Rust daemon; it's got code in
already for DAX, and apparently working with CrosVM which I believe are
based on my original proposals for the protocol.
But it would probably need a clean fix to this deadlock.

IMHO the slave channel needs properly specifying; no one really
understands what the semantics are supposed to be.

Dave

> > AFAIK the event loop is only needed for the VHOST_USER_GET_VRING_BASE
> > message. Another possibility might be to create the nested event loop
> > in this case only : this would allow VHOST_USER_GET_STATUS to complete
> > before QEMU starts processing the VHOST_USER_SLAVE_IOTLB_MSG requests.
> > 
> > Cheers,
> > 
> > --
> > Greg
> > 
> > > Maxime
> > > 
> > > [0]: 
> > > https://patchwork.ozlabs.org/project/qemu-devel/patch/20210312092212.782255-6-groug@kaod.org/
> > > 
> > > 
> > > > Break on first error message("Received unexpected msg type. Expected 22 
> > > > received 40")
> > > > 
> > > > #0  0x0000555555b72ed4 in process_message_reply (dev=0x5555584dd600, 
> > > > msg=0x7fffffffa330) at ../hw/virtio/vhost-user.c:445
> > > > #1  0x0000555555b77c26 in vhost_user_send_device_iotlb_msg 
> > > > (dev=0x5555584dd600, imsg=0x7fffffffa600) at 
> > > > ../hw/virtio/vhost-user.c:2341
> > > > #2  0x0000555555b7179e in vhost_backend_update_device_iotlb 
> > > > (dev=0x5555584dd600, iova=10442706944, uaddr=140736119902208, len=4096, 
> > > > perm=IOMMU_RW) at ../hw/virtio/vhost-backend.c:361
> > > > #3  0x0000555555b6e34c in vhost_device_iotlb_miss (dev=0x5555584dd600, 
> > > > iova=10442706944, write=1) at ../hw/virtio/vhost.c:1113
> > > > #4  0x0000555555b718d9 in vhost_backend_handle_iotlb_msg 
> > > > (dev=0x5555584dd600, imsg=0x7fffffffa7b0) at 
> > > > ../hw/virtio/vhost-backend.c:393
> > > > #5  0x0000555555b76144 in slave_read (ioc=0x555557a38680, 
> > > > condition=G_IO_IN, opaque=0x5555584dd600) at 
> > > > ../hw/virtio/vhost-user.c:1726
> > > > #6  0x0000555555c797a5 in qio_channel_fd_source_dispatch 
> > > > (source=0x555556a06fb0, callback=0x555555b75f86 <slave_read>, 
> > > > user_data=0x5555584dd600) at ../io/channel-watch.c:84
> > > > #7  0x00007ffff554895d in g_main_context_dispatch () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #8  0x00007ffff5548d18 in g_main_context_iterate.isra () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #9  0x00007ffff5549042 in g_main_loop_run () at /lib64/libglib-2.0.so.0
> > > > #10 0x0000555555b72de7 in vhost_user_read (dev=0x5555584dd600, 
> > > > msg=0x7fffffffac50) at ../hw/virtio/vhost-user.c:413
> > > > #11 0x0000555555b72e9b in process_message_reply (dev=0x5555584dd600, 
> > > > msg=0x7fffffffaf10) at ../hw/virtio/vhost-user.c:439
> > > > #12 0x0000555555b77c26 in vhost_user_send_device_iotlb_msg 
> > > > (dev=0x5555584dd600, imsg=0x7fffffffb1e0) at 
> > > > ../hw/virtio/vhost-user.c:2341
> > > > #13 0x0000555555b7179e in vhost_backend_update_device_iotlb 
> > > > (dev=0x5555584dd600, iova=10468392960, uaddr=140736145588224, len=4096, 
> > > > perm=IOMMU_RW) at ../hw/virtio/vhost-backend.c:361
> > > > #14 0x0000555555b6e34c in vhost_device_iotlb_miss (dev=0x5555584dd600, 
> > > > iova=10468392960, write=1) at ../hw/virtio/vhost.c:1113
> > > > #15 0x0000555555b718d9 in vhost_backend_handle_iotlb_msg 
> > > > (dev=0x5555584dd600, imsg=0x7fffffffb390) at 
> > > > ../hw/virtio/vhost-backend.c:393
> > > > #16 0x0000555555b76144 in slave_read (ioc=0x555557a38680, 
> > > > condition=G_IO_IN, opaque=0x5555584dd600) at 
> > > > ../hw/virtio/vhost-user.c:1726
> > > > #17 0x0000555555c797a5 in qio_channel_fd_source_dispatch 
> > > > (source=0x555556c70250, callback=0x555555b75f86 <slave_read>, 
> > > > user_data=0x5555584dd600) at ../io/channel-watch.c:84
> > > > #18 0x00007ffff554895d in g_main_context_dispatch () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #19 0x00007ffff5548d18 in g_main_context_iterate.isra () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #20 0x00007ffff5549042 in g_main_loop_run () at /lib64/libglib-2.0.so.0
> > > > #21 0x0000555555b72de7 in vhost_user_read (dev=0x5555584dd600, 
> > > > msg=0x7fffffffb830) at ../hw/virtio/vhost-user.c:413
> > > > #22 0x0000555555b72e9b in process_message_reply (dev=0x5555584dd600, 
> > > > msg=0x7fffffffbaf0) at ../hw/virtio/vhost-user.c:439
> > > > #23 0x0000555555b77c26 in vhost_user_send_device_iotlb_msg 
> > > > (dev=0x5555584dd600, imsg=0x7fffffffbdc0) at 
> > > > ../hw/virtio/vhost-user.c:2341
> > > > #24 0x0000555555b7179e in vhost_backend_update_device_iotlb 
> > > > (dev=0x5555584dd600, iova=10442702848, uaddr=140736119898112, len=4096, 
> > > > perm=IOMMU_RW) at ../hw/virtio/vhost-backend.c:361
> > > > #25 0x0000555555b6e34c in vhost_device_iotlb_miss (dev=0x5555584dd600, 
> > > > iova=10442702848, write=1) at ../hw/virtio/vhost.c:1113
> > > > #26 0x0000555555b718d9 in vhost_backend_handle_iotlb_msg 
> > > > (dev=0x5555584dd600, imsg=0x7fffffffbf70) at 
> > > > ../hw/virtio/vhost-backend.c:393
> > > > #27 0x0000555555b76144 in slave_read (ioc=0x555557a38680, 
> > > > condition=G_IO_IN, opaque=0x5555584dd600) at 
> > > > ../hw/virtio/vhost-user.c:1726
> > > > #28 0x0000555555c797a5 in qio_channel_fd_source_dispatch 
> > > > (source=0x555556f1a530, callback=0x555555b75f86 <slave_read>, 
> > > > user_data=0x5555584dd600) at ../io/channel-watch.c:84
> > > > #29 0x00007ffff554895d in g_main_context_dispatch () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #30 0x00007ffff5548d18 in g_main_context_iterate.isra () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #31 0x00007ffff5549042 in g_main_loop_run () at /lib64/libglib-2.0.so.0
> > > > #32 0x0000555555b72de7 in vhost_user_read (dev=0x5555584dd600, 
> > > > msg=0x7fffffffc420) at ../hw/virtio/vhost-user.c:413
> > > > #33 0x0000555555b754b1 in vhost_user_get_u64 (dev=0x5555584dd600, 
> > > > request=40, u64=0x7fffffffc6e0) at ../hw/virtio/vhost-user.c:1349
> > > > #34 0x0000555555b758ff in vhost_user_get_status (dev=0x5555584dd600, 
> > > > status=0x7fffffffc713 "W\020") at ../hw/virtio/vhost-user.c:1474
> > > > #35 0x0000555555b75967 in vhost_user_add_status (dev=0x5555584dd600, 
> > > > status=7 '\a') at ../hw/virtio/vhost-user.c:1488
> > > > #36 0x0000555555b78bf6 in vhost_user_dev_start (dev=0x5555584dd600, 
> > > > started=true) at ../hw/virtio/vhost-user.c:2758
> > > > #37 0x0000555555b709ad in vhost_dev_start (hdev=0x5555584dd600, 
> > > > vdev=0x555557b965d0, vrings=false) at ../hw/virtio/vhost.c:1988
> > > > #38 0x000055555584291c in vhost_net_start_one (net=0x5555584dd600, 
> > > > dev=0x555557b965d0) at ../hw/net/vhost_net.c:271
> > > > #39 0x0000555555842f1e in vhost_net_start (dev=0x555557b965d0, 
> > > > ncs=0x555557bc09e0, data_queue_pairs=1, cvq=0) at 
> > > > ../hw/net/vhost_net.c:412
> > > > #40 0x0000555555b1bf61 in virtio_net_vhost_status (n=0x555557b965d0, 
> > > > status=15 '\017') at ../hw/net/virtio-net.c:311
> > > > #41 0x0000555555b1c20c in virtio_net_set_status (vdev=0x555557b965d0, 
> > > > status=15 '\017') at ../hw/net/virtio-net.c:392
> > > > #42 0x0000555555b1ed04 in virtio_net_handle_mq (n=0x555557b965d0, cmd=0 
> > > > '\000', iov=0x555556c7ef50, iov_cnt=1) at ../hw/net/virtio-net.c:1497
> > > > #43 0x0000555555b1eef0 in virtio_net_handle_ctrl_iov 
> > > > (vdev=0x555557b965d0, in_sg=0x555556a09880, in_num=1, 
> > > > out_sg=0x555556a09890, out_num=1) at ../hw/net/virtio-net.c:1534
> > > > #44 0x0000555555b1efe9 in virtio_net_handle_ctrl (vdev=0x555557b965d0, 
> > > > vq=0x7fffc04ac140) at ../hw/net/virtio-net.c:1557
> > > > #45 0x0000555555b63776 in virtio_queue_notify_vq (vq=0x7fffc04ac140) at 
> > > > ../hw/virtio/virtio.c:2249
> > > > #46 0x0000555555b669dc in virtio_queue_host_notifier_read 
> > > > (n=0x7fffc04ac1b4) at ../hw/virtio/virtio.c:3529
> > > > #47 0x0000555555e3f458 in aio_dispatch_handler (ctx=0x555556a016c0, 
> > > > node=0x7ffd8800e430) at ../util/aio-posix.c:369
> > > > #48 0x0000555555e3f613 in aio_dispatch_handlers (ctx=0x555556a016c0) at 
> > > > ../util/aio-posix.c:412
> > > > #49 0x0000555555e3f669 in aio_dispatch (ctx=0x555556a016c0) at 
> > > > ../util/aio-posix.c:422
> > > > #50 0x0000555555e585de in aio_ctx_dispatch (source=0x555556a016c0, 
> > > > callback=0x0, user_data=0x0) at ../util/async.c:321
> > > > #51 0x00007ffff554895d in g_main_context_dispatch () at 
> > > > /lib64/libglib-2.0.so.0
> > > > #52 0x0000555555e5abea in glib_pollfds_poll () at 
> > > > ../util/main-loop.c:295
> > > > #53 0x0000555555e5ac64 in os_host_main_loop_wait (timeout=0) at 
> > > > ../util/main-loop.c:318
> > > > #54 0x0000555555e5ad69 in main_loop_wait (nonblocking=0) at 
> > > > ../util/main-loop.c:604
> > > > #55 0x00005555559693de in qemu_main_loop () at ../softmmu/runstate.c:731
> > > > #56 0x00005555556e7c06 in qemu_default_main () at ../softmmu/main.c:37
> > > > #57 0x00005555556e7c3c in main (argc=71, argv=0x7fffffffcda8) at 
> > > > ../softmmu/main.c:48
> > > > 
> > > > 
> > > > 
> > > > -----Original Message-----
> > > > From: Maxime Coquelin <maxime.coquelin@redhat.com>
> > > > Sent: Thursday, January 12, 2023 5:26 PM
> > > > To: Laurent Vivier <lvivier@redhat.com>
> > > > Cc: qemu-devel@nongnu.org; Peter Maydell <peter.maydell@linaro.org>; 
> > > > Yajun Wu <yajunw@nvidia.com>; Parav Pandit <parav@nvidia.com>; Michael 
> > > > S. Tsirkin <mst@redhat.com>
> > > > Subject: Re: [PULL v4 76/83] vhost-user: Support vhost_dev_start
> > > > 
> > > > External email: Use caution opening links or attachments
> > > > 
> > > > 
> > > > Hi Laurent,
> > > > 
> > > > On 1/11/23 10:50, Laurent Vivier wrote:
> > > >> On 1/9/23 11:55, Michael S. Tsirkin wrote:
> > > >>> On Fri, Jan 06, 2023 at 03:21:43PM +0100, Laurent Vivier wrote:
> > > >>>> Hi,
> > > >>>>
> > > >>>> it seems this patch breaks vhost-user with DPDK.
> > > >>>>
> > > >>>> See
> > > >>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fbu
> > > >>>> gzilla.redhat.com%2Fshow_bug.cgi%3Fid%3D2155173&data=05%7C01%7Cyajun
> > > >>>> w%40nvidia.com%7C47e6e0fabd044383fd3308daf47f0253%7C43083d15727340c1
> > > >>>> b7db39efd9ccc17a%7C0%7C0%7C638091123577559319%7CUnknown%7CTWFpbGZsb3
> > > >>>> d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D
> > > >>>> %7C3000%7C%7C%7C&sdata=1pjChYTKHVmBoempNitiZHBdrlPIMFjKoD6FeOVSay0%3
> > > >>>> D&reserved=0
> > > >>>>
> > > >>>> it seems QEMU doesn't receive the expected commands sequence:
> > > >>>>
> > > >>>> Received unexpected msg type. Expected 22 received 40 Fail to update
> > > >>>> device iotlb Received unexpected msg type. Expected 40 received 22
> > > >>>> Received unexpected msg type. Expected 22 received 11 Fail to update
> > > >>>> device iotlb Received unexpected msg type. Expected 11 received 22
> > > >>>> vhost VQ 1 ring restore failed: -71: Protocol error (71) Received
> > > >>>> unexpected msg type. Expected 22 received 11 Fail to update device
> > > >>>> iotlb Received unexpected msg type. Expected 11 received 22 vhost VQ
> > > >>>> 0 ring restore failed: -71: Protocol error (71) unable to start
> > > >>>> vhost net: 71: falling back on userspace virtio
> > > >>>>
> > > >>>> It receives VHOST_USER_GET_STATUS (40) when it expects
> > > >>>> VHOST_USER_IOTLB_MSG (22) and VHOST_USER_IOTLB_MSG when it expects
> > > >>>> VHOST_USER_GET_STATUS.
> > > >>>> and VHOST_USER_GET_VRING_BASE (11) when it expect
> > > >>>> VHOST_USER_GET_STATUS and so on.
> > > >>>>
> > > >>>> Any idea?
> > > > 
> > > > We only have a single thread on DPDK side to handle Vhost-user 
> > > > requests, it will read a request, handle it and reply to it. Then it 
> > > > reads the next one, etc... So I don't think it is possible to mix 
> > > > request replies order on DPDK side.
> > > > 
> > > > Maybe there are two threads concurrently sending requests on QEMU side?
> > > > 
> > > > Regards,
> > > > Maxime
> > > > 
> > > >>>> Thanks,
> > > >>>> Laurent
> > > >>>
> > > >>>
> > > >>> So I am guessing it's coming from:
> > > >>>
> > > >>>       if (msg.hdr.request != request) {
> > > >>>           error_report("Received unexpected msg type. Expected %d
> > > >>> received %d",
> > > >>>                        request, msg.hdr.request);
> > > >>>           return -EPROTO;
> > > >>>       }
> > > >>>
> > > >>> in process_message_reply and/or in vhost_user_get_u64.
> > > >>>
> > > >>>
> > > >>>> On 11/7/22 23:53, Michael S. Tsirkin wrote:
> > > >>>>> From: Yajun Wu <yajunw@nvidia.com>
> > > >>>>>
> > > >>>>> The motivation of adding vhost-user vhost_dev_start support is to
> > > >>>>> improve backend configuration speed and reduce live migration VM
> > > >>>>> downtime.
> > > >>>>>
> > > >>>>> Today VQ configuration is issued one by one. For virtio net with
> > > >>>>> multi-queue support, backend needs to update RSS (Receive side
> > > >>>>> scaling) on every rx queue enable. Updating RSS is time-consuming
> > > >>>>> (typical time like 7ms).
> > > >>>>>
> > > >>>>> Implement already defined vhost status and message in the vhost
> > > >>>>> specification [1].
> > > >>>>> (a) VHOST_USER_PROTOCOL_F_STATUS
> > > >>>>> (b) VHOST_USER_SET_STATUS
> > > >>>>> (c) VHOST_USER_GET_STATUS
> > > >>>>>
> > > >>>>> Send message VHOST_USER_SET_STATUS with VIRTIO_CONFIG_S_DRIVER_OK
> > > >>>>> for device start and reset(0) for device stop.
> > > >>>>>
> > > >>>>> On reception of the DRIVER_OK message, backend can apply the needed
> > > >>>>> setting only once (instead of incremental) and also utilize
> > > >>>>> parallelism on enabling queues.
> > > >>>>>
> > > >>>>> This improves QEMU's live migration downtime with vhost user
> > > >>>>> backend implementation by great margin, specially for the large
> > > >>>>> number of VQs of 64 from 800 msec to 250 msec.
> > > >>>>>
> > > >>>>> [1]
> > > >>>>> https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Fq
> > > >>>>> emu-project.gitlab.io%2Fqemu%2Finterop%2Fvhost-user.html&data=05%7C
> > > >>>>> 01%7Cyajunw%40nvidia.com%7C47e6e0fabd044383fd3308daf47f0253%7C43083
> > > >>>>> d15727340c1b7db39efd9ccc17a%7C0%7C0%7C638091123577559319%7CUnknown%
> > > >>>>> 7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiL
> > > >>>>> CJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=YPbrFRJA92KLLwADMUDvwBt%2Fme2Ef
> > > >>>>> GZuVANOmXH5pic%3D&reserved=0
> > > >>>>>
> > > >>>>> Signed-off-by: Yajun Wu <yajunw@nvidia.com>
> > > >>>>> Acked-by: Parav Pandit <parav@nvidia.com>
> > > >>>>> Message-Id: <20221017064452.1226514-3-yajunw@nvidia.com>
> > > >>>>> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
> > > >>>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > >>>
> > > >>> Probably easiest to debug from dpdk side.
> > > >>> Does the problem go away if you disable the feature
> > > >>> VHOST_USER_PROTOCOL_F_STATUS in dpdk?
> > > >>
> > > >> Maxime could you help to debug this?
> > > >>
> > > >> Thanks,
> > > >> Laurent
> > > >>
> > > > 
> > > 
> > 
> > 
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK