From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Wed, 12 Apr 2023 16:51:38 -0400 From: Stefan Hajnoczi Message-ID: <20230412205138.GA2813183@fedora> References: <20230411150515.14020-1-hreitz@redhat.com> <20230411150515.14020-2-hreitz@redhat.com> MIME-Version: 1.0 Content-Type: multipart/signed; micalg=pgp-sha256; protocol="application/pgp-signature"; boundary="rRJT1mSd1+ho7w0n" Content-Disposition: inline In-Reply-To: <20230411150515.14020-2-hreitz@redhat.com> Subject: Re: [Virtio-fs] [PATCH 1/4] vhost: Re-enable vrings after setting features List-Id: Development discussions about virtio-fs List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Hanna Czenczek Cc: qemu-devel@nongnu.org, virtio-fs@redhat.com, German Maglione , Anton Kuchin , Juan Quintela , "Michael S . Tsirkin" , Stefano Garzarella , Roman Kagan , Maxime Coquelin , =?iso-8859-1?Q?Marc-Andr=E9?= Lureau --rRJT1mSd1+ho7w0n Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: quoted-printable On Tue, Apr 11, 2023 at 05:05:12PM +0200, Hanna Czenczek wrote: > If the back-end supports the VHOST_USER_F_PROTOCOL_FEATURES feature, > setting the vhost features will set this feature, too. Doing so > disables all vrings, which may not be intended. >=20 > For example, enabling or disabling logging during migration requires > setting those features (to set or unset VHOST_F_LOG_ALL), which will > automatically disable all vrings. In either case, the VM is running > (disabling logging is done after a failed or cancelled migration, and > only once the VM is running again, see comment in > memory_global_dirty_log_stop()), so the vrings should really be enabled. > As a result, the back-end seems to hang. >=20 > To fix this, we must remember whether the vrings are supposed to be > enabled, and, if so, re-enable them after a SET_FEATURES call that set > VHOST_USER_F_PROTOCOL_FEATURES. >=20 > It seems less than ideal that there is a short period in which the VM is > running but the vrings will be stopped (between SET_FEATURES and > SET_VRING_ENABLE). To fix this, we would need to change the protocol, > e.g. by introducing a new flag or vhost-user protocol feature to disable > disabling vrings whenever VHOST_USER_F_PROTOCOL_FEATURES is set, or add > new functions for setting/clearing singular feature bits (so that > F_LOG_ALL can be set/cleared without touching F_PROTOCOL_FEATURES). >=20 > Even with such a potential addition to the protocol, we still need this > fix here, because we cannot expect that back-ends will implement this > addition. >=20 > Signed-off-by: Hanna Czenczek > --- > include/hw/virtio/vhost.h | 10 ++++++++++ > hw/virtio/vhost.c | 13 +++++++++++++ > 2 files changed, 23 insertions(+) >=20 > diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h > index a52f273347..2fe02ed5d4 100644 > --- a/include/hw/virtio/vhost.h > +++ b/include/hw/virtio/vhost.h > @@ -90,6 +90,16 @@ struct vhost_dev { > int vq_index_end; > /* if non-zero, minimum required value for max_queues */ > int num_queues; > + > + /* > + * Whether the virtqueues are supposed to be enabled (via > + * SET_VRING_ENABLE). Setting the features (e.g. for > + * enabling/disabling logging) will disable all virtqueues if > + * VHOST_USER_F_PROTOCOL_FEATURES is set, so then we need to > + * re-enable them if this field is set. > + */ > + bool enable_vqs; > + > /** > * vhost feature handling requires matching the feature set > * offered by a backend which may be a subset of the total > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c > index a266396576..cbff589efa 100644 > --- a/hw/virtio/vhost.c > +++ b/hw/virtio/vhost.c > @@ -50,6 +50,8 @@ static unsigned int used_memslots; > static QLIST_HEAD(, vhost_dev) vhost_devices =3D > QLIST_HEAD_INITIALIZER(vhost_devices); > =20 > +static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable= ); > + > bool vhost_has_free_slot(void) > { > unsigned int slots_limit =3D ~0U; > @@ -899,6 +901,15 @@ static int vhost_dev_set_features(struct vhost_dev *= dev, > } > } > =20 > + if (dev->enable_vqs) { > + /* > + * Setting VHOST_USER_F_PROTOCOL_FEATURES would have disabled all > + * virtqueues, even if that was not intended; re-enable them if > + * necessary. > + */ > + vhost_dev_set_vring_enable(dev, true); > + } > + > out: > return r; > } > @@ -1896,6 +1907,8 @@ int vhost_dev_get_inflight(struct vhost_dev *dev, u= int16_t queue_size, > =20 > static int vhost_dev_set_vring_enable(struct vhost_dev *hdev, int enable) > { > + hdev->enable_vqs =3D enable; > + > if (!hdev->vhost_ops->vhost_set_vring_enable) { > return 0; > } The vhost-user spec doesn't say that VHOST_F_LOG_ALL needs to be toggled at runtime and I don't think VHOST_USER_SET_PROTOCOL_FEATURES is intended to be used like that. This issue shows why doing so is a bad idea. VHOST_F_LOG_ALL does not need to be toggled to control logging. Logging is controlled at runtime by the presence of the dirty log (VHOST_USER_SET_LOG_BASE) and the per-vring logging flag (VHOST_VRING_F_LOG). I suggest permanently enabling VHOST_F_LOG_ALL upon connection when the the backend supports it. No spec changes are required. libvhost-user looks like it will work. I didn't look at DPDK/SPDK, but checking that it works there is important too. I have CCed people who may be interested in this issue. This is the first time I've looked at vhost-user logging, so this idea may not work. 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