From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-ID: Date: Thu, 13 Apr 2023 19:55:00 +0200 MIME-Version: 1.0 References: <20230411150515.14020-1-hreitz@redhat.com> <20230411150515.14020-3-hreitz@redhat.com> <20230412210641.GC2813183@fedora> <246691c3-2287-7472-3615-7ec0f074445c@redhat.com> From: Hanna Czenczek In-Reply-To: Content-Language: en-US Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Transfer-Encoding: 8bit Subject: Re: [Virtio-fs] [PATCH 2/4] vhost-user: Interface for migration state transfer List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: Stefan Hajnoczi , qemu-devel@nongnu.org, virtio-fs@redhat.com, German Maglione , Anton Kuchin , Juan Quintela , "Michael S . Tsirkin" , Stefano Garzarella , =?UTF-8?Q?Eugenio_P=c3=a9rez?= On 13.04.23 13:38, Stefan Hajnoczi wrote: > On Thu, 13 Apr 2023 at 05:24, Hanna Czenczek wrote: >> On 12.04.23 23:06, Stefan Hajnoczi wrote: >>> On Tue, Apr 11, 2023 at 05:05:13PM +0200, Hanna Czenczek wrote: >>>> So-called "internal" virtio-fs migration refers to transporting the >>>> back-end's (virtiofsd's) state through qemu's migration stream. To do >>>> this, we need to be able to transfer virtiofsd's internal state to and >>>> from virtiofsd. >>>> >>>> Because virtiofsd's internal state will not be too large, we believe it >>>> is best to transfer it as a single binary blob after the streaming >>>> phase. Because this method should be useful to other vhost-user >>>> implementations, too, it is introduced as a general-purpose addition to >>>> the protocol, not limited to vhost-user-fs. >>>> >>>> These are the additions to the protocol: >>>> - New vhost-user protocol feature VHOST_USER_PROTOCOL_F_MIGRATORY_STATE: >>>> This feature signals support for transferring state, and is added so >>>> that migration can fail early when the back-end has no support. >>>> >>>> - SET_DEVICE_STATE_FD function: Front-end and back-end negotiate a pipe >>>> over which to transfer the state. The front-end sends an FD to the >>>> back-end into/from which it can write/read its state, and the back-end >>>> can decide to either use it, or reply with a different FD for the >>>> front-end to override the front-end's choice. >>>> The front-end creates a simple pipe to transfer the state, but maybe >>>> the back-end already has an FD into/from which it has to write/read >>>> its state, in which case it will want to override the simple pipe. >>>> Conversely, maybe in the future we find a way to have the front-end >>>> get an immediate FD for the migration stream (in some cases), in which >>>> case we will want to send this to the back-end instead of creating a >>>> pipe. >>>> Hence the negotiation: If one side has a better idea than a plain >>>> pipe, we will want to use that. >>>> >>>> - CHECK_DEVICE_STATE: After the state has been transferred through the >>>> pipe (the end indicated by EOF), the front-end invokes this function >>>> to verify success. There is no in-band way (through the pipe) to >>>> indicate failure, so we need to check explicitly. >>>> >>>> Once the transfer pipe has been established via SET_DEVICE_STATE_FD >>>> (which includes establishing the direction of transfer and migration >>>> phase), the sending side writes its data into the pipe, and the reading >>>> side reads it until it sees an EOF. Then, the front-end will check for >>>> success via CHECK_DEVICE_STATE, which on the destination side includes >>>> checking for integrity (i.e. errors during deserialization). >>>> >>>> Suggested-by: Stefan Hajnoczi >>>> Signed-off-by: Hanna Czenczek >>>> --- >>>> include/hw/virtio/vhost-backend.h | 24 +++++ >>>> include/hw/virtio/vhost.h | 79 ++++++++++++++++ >>>> hw/virtio/vhost-user.c | 147 ++++++++++++++++++++++++++++++ >>>> hw/virtio/vhost.c | 37 ++++++++ >>>> 4 files changed, 287 insertions(+) >>>> >>>> diff --git a/include/hw/virtio/vhost-backend.h b/include/hw/virtio/vhost-backend.h >>>> index ec3fbae58d..5935b32fe3 100644 >>>> --- a/include/hw/virtio/vhost-backend.h >>>> +++ b/include/hw/virtio/vhost-backend.h >>>> @@ -26,6 +26,18 @@ typedef enum VhostSetConfigType { >>>> VHOST_SET_CONFIG_TYPE_MIGRATION = 1, >>>> } VhostSetConfigType; >>>> >>>> +typedef enum VhostDeviceStateDirection { >>>> + /* Transfer state from back-end (device) to front-end */ >>>> + VHOST_TRANSFER_STATE_DIRECTION_SAVE = 0, >>>> + /* Transfer state from front-end to back-end (device) */ >>>> + VHOST_TRANSFER_STATE_DIRECTION_LOAD = 1, >>>> +} VhostDeviceStateDirection; >>>> + >>>> +typedef enum VhostDeviceStatePhase { >>>> + /* The device (and all its vrings) is stopped */ >>>> + VHOST_TRANSFER_STATE_PHASE_STOPPED = 0, >>>> +} VhostDeviceStatePhase; >>> vDPA has: >>> >>> /* Suspend a device so it does not process virtqueue requests anymore >>> * >>> * After the return of ioctl the device must preserve all the necessary state >>> * (the virtqueue vring base plus the possible device specific states) that is >>> * required for restoring in the future. The device must not change its >>> * configuration after that point. >>> */ >>> #define VHOST_VDPA_SUSPEND _IO(VHOST_VIRTIO, 0x7D) >>> >>> /* Resume a device so it can resume processing virtqueue requests >>> * >>> * After the return of this ioctl the device will have restored all the >>> * necessary states and it is fully operational to continue processing the >>> * virtqueue descriptors. >>> */ >>> #define VHOST_VDPA_RESUME _IO(VHOST_VIRTIO, 0x7E) >>> >>> I wonder if it makes sense to import these into vhost-user so that the >>> difference between kernel vhost and vhost-user is minimized. It's okay >>> if one of them is ahead of the other, but it would be nice to avoid >>> overlapping/duplicated functionality. >>> >>> (And I hope vDPA will import the device state vhost-user messages >>> introduced in this series.) >> I don’t understand your suggestion. (Like, I very simply don’t >> understand :)) >> >> These are vhost messages, right? What purpose do you have in mind for >> them in vhost-user for internal migration? They’re different from the >> state transfer messages, because they don’t transfer state to/from the >> front-end. Also, the state transfer stuff is supposed to be distinct >> from starting/stopping the device; right now, it just requires the >> device to be stopped beforehand (or started only afterwards). And in >> the future, new VhostDeviceStatePhase values may allow the messages to >> be used on devices that aren’t stopped. >> >> So they seem to serve very different purposes. I can imagine using the >> VDPA_{SUSPEND,RESUME} messages for external migration (what Anton is >> working on), but they don’t really help with internal migration >> implemented here. If I were to add them, they’d just be sent in >> addition to the new messages added in this patch here, i.e. SUSPEND on >> the source before SET_DEVICE_STATE_FD, and RESUME on the destination >> after CHECK_DEVICE_STATE (we could use RESUME in place of >> CHECK_DEVICE_STATE on the destination, but we can’t do that on the >> source, so we still need CHECK_DEVICE_STATE). > Yes, they are complementary to the device state fd message. I want to > make sure pre-conditions about the device's state (running vs stopped) > already take into account the vDPA SUSPEND/RESUME model. > > vDPA will need device state save/load in the future. For virtiofs > devices, for example. This is why I think we should plan for vDPA and > vhost-user to share the same interface. While the paragraph below is more important, I don’t feel like this would be important right now.  It’s clear that SUSPEND must come before transferring any state, and that RESUME must come after transferring state.  I don’t think we need to clarify this now, it’d be obvious when implementing SUSPEND/RESUME. > Also, I think the code path you're relying on (vhost_dev_stop()) on > doesn't work for backends that implement VHOST_USER_PROTOCOL_F_STATUS > because stopping the backend resets the device and throws away its > state. SUSPEND/RESUME solve this. This looks like a more general > problem since vhost_dev_stop() is called any time the VM is paused. > Maybe it needs to use SUSPEND/RESUME whenever possible. That’s a problem.  Quite a problem, to be honest, because this sounds rather complicated with honestly absolutely no practical benefit right now. Would you require SUSPEND/RESUME for state transfer even if the back-end does not implement GET/SET_STATUS?  Because then this would also lead to more complexity in virtiofsd. Basically, what I’m hearing is that I need to implement a different feature that has no practical impact right now, and also fix bugs around it along the way... (Not that I have any better suggestion.) Hanna