From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Message-ID: <44fac0de-d651-1b1b-e263-161587c40d3b@redhat.com> Date: Thu, 13 Apr 2023 11:04:29 +0200 MIME-Version: 1.0 References: <20230411150515.14020-1-hreitz@redhat.com> <20230411150515.14020-4-hreitz@redhat.com> <20230412211447.GD2813183@fedora> From: Hanna Czenczek In-Reply-To: <20230412211447.GD2813183@fedora> Content-Language: en-US Content-Type: text/plain; charset="utf-8"; format="flowed" Content-Transfer-Encoding: 8bit Subject: Re: [Virtio-fs] [PATCH 3/4] vhost: Add high-level state save/load functions List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Stefan Hajnoczi Cc: qemu-devel@nongnu.org, virtio-fs@redhat.com, German Maglione , Anton Kuchin , Juan Quintela , "Michael S . Tsirkin" , Stefano Garzarella On 12.04.23 23:14, Stefan Hajnoczi wrote: > On Tue, Apr 11, 2023 at 05:05:14PM +0200, Hanna Czenczek wrote: >> vhost_save_backend_state() and vhost_load_backend_state() can be used by >> vhost front-ends to easily save and load the back-end's state to/from >> the migration stream. >> >> Because we do not know the full state size ahead of time, >> vhost_save_backend_state() simply reads the data in 1 MB chunks, and >> writes each chunk consecutively into the migration stream, prefixed by >> its length. EOF is indicated by a 0-length chunk. >> >> Signed-off-by: Hanna Czenczek >> --- >> include/hw/virtio/vhost.h | 35 +++++++ >> hw/virtio/vhost.c | 196 ++++++++++++++++++++++++++++++++++++++ >> 2 files changed, 231 insertions(+) >> >> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h >> index 29449e0fe2..d1f1e9e1f3 100644 >> --- a/include/hw/virtio/vhost.h >> +++ b/include/hw/virtio/vhost.h >> @@ -425,4 +425,39 @@ int vhost_set_device_state_fd(struct vhost_dev *dev, >> */ >> int vhost_check_device_state(struct vhost_dev *dev, Error **errp); >> >> +/** >> + * vhost_save_backend_state(): High-level function to receive a vhost >> + * back-end's state, and save it in `f`. Uses >> + * `vhost_set_device_state_fd()` to get the data from the back-end, and >> + * stores it in consecutive chunks that are each prefixed by their >> + * respective length (be32). The end is marked by a 0-length chunk. >> + * >> + * Must only be called while the device and all its vrings are stopped >> + * (`VHOST_TRANSFER_STATE_PHASE_STOPPED`). >> + * >> + * @dev: The vhost device from which to save the state >> + * @f: Migration stream in which to save the state >> + * @errp: Potential error message >> + * >> + * Returns 0 on success, and -errno otherwise. >> + */ >> +int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp); >> + >> +/** >> + * vhost_load_backend_state(): High-level function to load a vhost >> + * back-end's state from `f`, and send it over to the back-end. Reads >> + * the data from `f` in the format used by `vhost_save_state()`, and >> + * uses `vhost_set_device_state_fd()` to transfer it to the back-end. >> + * >> + * Must only be called while the device and all its vrings are stopped >> + * (`VHOST_TRANSFER_STATE_PHASE_STOPPED`). >> + * >> + * @dev: The vhost device to which to send the sate >> + * @f: Migration stream from which to load the state >> + * @errp: Potential error message >> + * >> + * Returns 0 on success, and -errno otherwise. >> + */ >> +int vhost_load_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp); >> + >> #endif >> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c >> index 90099d8f6a..d08849c691 100644 >> --- a/hw/virtio/vhost.c >> +++ b/hw/virtio/vhost.c >> @@ -2125,3 +2125,199 @@ int vhost_check_device_state(struct vhost_dev *dev, Error **errp) >> "vhost transport does not support migration state transfer"); >> return -ENOSYS; >> } >> + >> +int vhost_save_backend_state(struct vhost_dev *dev, QEMUFile *f, Error **errp) >> +{ >> + /* Maximum chunk size in which to transfer the state */ >> + const size_t chunk_size = 1 * 1024 * 1024; >> + void *transfer_buf = NULL; >> + g_autoptr(GError) g_err = NULL; >> + int pipe_fds[2], read_fd = -1, write_fd = -1, reply_fd = -1; >> + int ret; >> + >> + /* [0] for reading (our end), [1] for writing (back-end's end) */ >> + if (!g_unix_open_pipe(pipe_fds, FD_CLOEXEC, &g_err)) { >> + error_setg(errp, "Failed to set up state transfer pipe: %s", >> + g_err->message); >> + ret = -EINVAL; >> + goto fail; >> + } >> + >> + read_fd = pipe_fds[0]; >> + write_fd = pipe_fds[1]; >> + >> + /* VHOST_TRANSFER_STATE_PHASE_STOPPED means the device must be stopped */ >> + assert(!dev->started && !dev->enable_vqs); >> + >> + /* Transfer ownership of write_fd to the back-end */ >> + ret = vhost_set_device_state_fd(dev, >> + VHOST_TRANSFER_STATE_DIRECTION_SAVE, >> + VHOST_TRANSFER_STATE_PHASE_STOPPED, >> + write_fd, >> + &reply_fd, >> + errp); >> + if (ret < 0) { >> + error_prepend(errp, "Failed to initiate state transfer: "); >> + goto fail; >> + } >> + >> + /* If the back-end wishes to use a different pipe, switch over */ >> + if (reply_fd >= 0) { >> + close(read_fd); >> + read_fd = reply_fd; >> + } >> + >> + transfer_buf = g_malloc(chunk_size); >> + >> + while (true) { >> + ssize_t read_ret; >> + >> + read_ret = read(read_fd, transfer_buf, chunk_size); >> + if (read_ret < 0) { >> + ret = -errno; >> + error_setg_errno(errp, -ret, "Failed to receive state"); >> + goto fail; >> + } >> + >> + assert(read_ret <= chunk_size); >> + qemu_put_be32(f, read_ret); >> + >> + if (read_ret == 0) { >> + /* EOF */ >> + break; >> + } >> + >> + qemu_put_buffer(f, transfer_buf, read_ret); >> + } > I think this synchronous approach with a single contiguous stream of > chunks is okay for now. > > Does this make the QEMU monitor unresponsive if the backend is slow? Oh, absolutely.  But as far as I can tell that’s also the case if the back-end doesn’t respond (or responds slowly) to vhost-user messages, because they’re generally sent/received synchronously. (So, notably, these synchronous read()/write() calls aren’t worse than the previous model of transferring data through shared memory, but awaiting the back-end via vhost-user calls between each chunk.) I don’t know whether it’s even possible to do it better (while keeping it all in the switch-over phase).  The VMState functions aren’t coroutines or AIO, so I can’t think of a way to improve this. (Well, except:) > In the future the interface could be extended to allow participating in > the iterative phase of migration. Then chunks from multiple backends > (plus guest RAM) would be interleaved and there would be some > parallelism. Sure.  That would also definitely help with an unintentionally slow back-end.  If the back-end making qemu unresponsive is a deal-breaker, then we’d have to do this now. Hanna