"Michael S. Tsirkin" <m...@redhat.com> writes:
> On Mon, Nov 21, 2022 at 07:15:30PM +0000, Alex Bennée wrote:
>>
>> Stefan Hajnoczi <stefa...@gmail.com> writes:
>>
>> > On Mon, 21 Nov 2022 at 09:49, Alex Bennée <alex.ben...@linaro.org> wrote:
>> >>
>> >> We have a bunch of variables associated with the device and the vhost
>> >> backend which are used inconsistently throughout the code base. Lets
>> >> start trying to bring some order by agreeing what each variable is
>> >> for. Some cases to address (vho/vio renames to avoid ambiguous results
>> >> while grepping):
>> >>
>> >> virtio->guest_features is mostly the live status of the features field
>> >> and read and written as such by the guest. It does get manipulated by
>> >> the various load state via virtio_set_features_nocheck(vdev, val) for
>> >> migration.
>> >>
>> >> virtio->host_features is the result of vcd->get_features() most of the
>> >> time and for vhost-user devices eventually ends up down at the vhost
>> >> get features message:
>> >>
>> >> ./hw/virtio/virtio-bus.c:66: vdev->host_features =
>> >> vdc->get_features(vdev, vdev->host_features,
>> >>
>> >> However virtio-net does a lot of direct modification of it:
>> >>
>> >> ./hw/net/virtio-net.c:3517: n->host_features |= (1ULL <<
>> >> VIRTIO_NET_F_MTU);
>> >> ./hw/net/virtio-net.c:3529: n->host_features |= (1ULL <<
>> >> VIRTIO_NET_F_SPEED_DUPLEX);
>> >> ./hw/net/virtio-net.c:3539: n->host_features |= (1ULL <<
>> >> VIRTIO_NET_F_SPEED_DUPLEX);
>> >> ./hw/net/virtio-net.c:3548: n->host_features |= (1ULL <<
>> >> VIRTIO_NET_F_STANDBY);
>> >> ./hw/virtio/virtio.c:3438: bool bad = (val & ~(vdev->host_features))
>> >> != 0;
>> >>
>> >> And we have this case which propagates the global QMP values for the
>> >> device to the host features. This causes the resent regression of
>> >> vhost-user-sock due to 69e1c14aa2 (virtio: core: vq reset feature
>> >> negotation support) because the reset feature was rejected by the
>> >> vhost-user backend causing it to freeze:
>> >>
>> >> ./hw/virtio/virtio.c:4667: status->host_features =
>> >> qmp_decode_features(vdev->device_id,
>> >>
>> >> virtio->backend_features is only used by virtio-net to stash the
>> >> vhost_net_get_features features for checking later:
>> >>
>> >> features = vhost_net_get_features(get_vhost_net(nc->peer), features);
>> >> vdev->vio_backend_features = features;
>> >>
>> >> and:
>> >>
>> >> if (n->mtu_bypass_backend &&
>> >> !virtio_has_feature(vdev->vio_backend_features,
>> >> VIRTIO_NET_F_MTU)) {
>> >> features &= ~(1ULL << VIRTIO_NET_F_MTU);
>> >> }
>> >>
>> >> vhost_dev->acked_features seems to mostly reflect
>> >> virtio->guest_features (but where in the negotiation cycle?). Except
>> >> for vhost_net where is becomes vhost_dev->backend_features
>> >>
>> >> ./backends/vhost-user.c:87: b->dev.vho_acked_features =
>> >> b->vdev->guest_features;
>> >> ./hw/block/vhost-user-blk.c:149: s->dev.vho_acked_features =
>> >> vdev->guest_features;
>> >> ./hw/net/vhost_net.c:132: net->dev.vho_acked_features =
>> >> net->dev.vho_backend_features;
>> >> ./hw/scsi/vhost-scsi-common.c:53: vsc->dev.vho_acked_features =
>> >> vdev->guest_features;
>> >> ./hw/virtio/vhost-user-fs.c:77: fs->vhost_dev.vho_acked_features =
>> >> vdev->guest_features;
>> >> ./hw/virtio/vhost-user-i2c.c:46: i2c->vhost_dev.vho_acked_features =
>> >> vdev->guest_features;
>> >> ./hw/virtio/vhost-user-rng.c:44: rng->vhost_dev.vho_acked_features =
>> >> vdev->guest_features;
>> >> ./hw/virtio/vhost-vsock-common.c:71:
>> >> vvc->vhost_dev.vho_acked_features = vdev->guest_features;
>> >> ./hw/virtio/vhost.c:1631: hdev->vho_acked_features |=
>> >> bit_mask;
>> >>
>> >> vhost_dev->backend_features has become overloaded with two use cases:
>> >>
>> >> ./hw/block/vhost-user-blk.c:336: s->dev.vho_backend_features = 0;
>> >> ./hw/net/vhost_net.c:180: net->dev.vho_backend_features =
>> >> qemu_has_vnet_hdr(options->net_backend)
>> >> ./hw/net/vhost_net.c:185: net->dev.vho_backend_features = 0;
>> >> ./hw/scsi/vhost-scsi.c:220: vsc->dev.vho_backend_features = 0;
>> >> ./hw/scsi/vhost-user-scsi.c:121: vsc->dev.vho_backend_features = 0;
>> >> ./hw/virtio/vhost-user.c:2083: dev->vho_backend_features |= 1ULL
>> >> << VHOST_USER_F_PROTOCOL_FEATURES;
>> >> One use for saving the availability of a vhost-net feature and another
>> >> for ensuring we add the protocol feature negotiation bit when querying
>> >> a vhost backend. Maybe the places where this is queried should really
>> >> be bools that can be queried in the appropriate places?
>> >>
>> >> Signed-off-by: Alex Bennée <alex.ben...@linaro.org>
>> >> Cc: Stefano Garzarella <sgarz...@redhat.com>
>> >> Cc: "Michael S. Tsirkin" <m...@redhat.com>
>> >> Cc: Stefan Hajnoczi <stefa...@gmail.com>
>> >> ---
>> >> include/hw/virtio/vhost.h | 18 +++++++++++++++---
>> >> include/hw/virtio/virtio.h | 20 +++++++++++++++++++-
>> >> 2 files changed, 34 insertions(+), 4 deletions(-)
>> >>
>> >> diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
>> >> index 353252ac3e..502aa5677a 100644
>> >> --- a/include/hw/virtio/vhost.h
>> >> +++ b/include/hw/virtio/vhost.h
>> >> @@ -88,13 +88,25 @@ struct vhost_dev {
>> >> int vq_index_end;
>> >> /* if non-zero, minimum required value for max_queues */
>> >> int num_queues;
>> >> + /**
>> >> + * vhost feature handling requires matching the feature set
>> >> + * offered by a backend which may be a subset of the total
>> >> + * features eventually offered to the guest.
>> >> + *
>> >> + * @features: available features provided by the backend
>> >> + * @acked_features: final set of negotiated features with the
>> >> + * front-end driver
>> >> + * @backend_features: additional feature bits applied during
>> >> negotiation
>> >
>> > What does this mean?
>>
>> Well practically it is currently either applying
>> VHOST_USER_F_PROTOCOL_FEATURES to the vhost_user_set_features() or
>> storing VHOST_NET_F_VIRTIO_NET_HDR which I think eventually gets applied
>> to:
>>
>> net->dev.acked_features = net->dev.backend_features;
>>
>> I suspect both could be dropped and handled as flags and applied at the
>> destination.
>>
>> >
>> >> + *
>> >> + * Finally the @protocol_features is the final protocal feature
>> >
>> > s/protocal/protocol/
>> >
>> > All the other fields are VIRTIO feature bits and this field holds the
>> > VHOST_USER_SET_FEATURES feature bits?
>>
>> No these are the protocol features so a totally separate set of feature
>> bits for the vhost user protocol. I don't think these apply to kernel
>> vhost stuff?
>>
>> >
>> >> + * set negotiated between QEMU and the backend (after
>> >> + * VHOST_USER_F_PROTOCOL_FEATURES is negotiated)
>> >> + */
>> >> uint64_t features;
>> >> - /** @acked_features: final set of negotiated features */
>> >> uint64_t acked_features;
>> >> - /** @backend_features: backend specific feature bits */
>> >> uint64_t backend_features;
>> >> - /** @protocol_features: final negotiated protocol features */
>> >> uint64_t protocol_features;
>> >> +
>> >> uint64_t max_queues;
>> >> uint64_t backend_cap;
>> >> /* @started: is the vhost device started? */
>> >> diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
>> >> index a973811cbf..9939a0a632 100644
>> >> --- a/include/hw/virtio/virtio.h
>> >> +++ b/include/hw/virtio/virtio.h
>> >> @@ -93,6 +93,12 @@ enum virtio_device_endian {
>> >> VIRTIO_DEVICE_ENDIAN_BIG,
>> >> };
>> >>
>> >> +/**
>> >> + * struct VirtIODevice - common VirtIO structure
>> >> + * @name: name of the device
>> >> + * @status: VirtIO Device Status field
>> >> + *
>> >> + */
>> >> struct VirtIODevice
>> >> {
>> >> DeviceState parent_obj;
>> >> @@ -100,9 +106,21 @@ struct VirtIODevice
>> >> uint8_t status;
>> >> uint8_t isr;
>> >> uint16_t queue_sel;
>> >> - uint64_t guest_features;
>> >> + /**
>> >> + * These fields represent a set of VirtIO features at various
>> >> + * levels of the stack. @host_features indicates the complete
>> >> + * feature set the VirtIO device can offer to the driver.
>> >> + * @guest_features indicates which features the VirtIO driver can
>> >> + * support.
>> >
>> > The device never knows the features that the driver can support, so
>> > this sentence is ambiguous/incorrect. The device only knows the
>> > features that the driver writes during negotiation, which the spec
>> > says is a subset of host_features.
>> >
>> > Maybe "indicates the features that driver wrote"?
>> >
>> > I noticed that this field is assigned even when the guest writes
>> > invalid feature bits.
>>
>> Should we fix that? The negotiation sequence should be guest read, mask
>> and write back so the value should be validated against host_features?
>>
>> >
>> >> Finally @backend_features represents everything
>> >> + * supported by the backend. This set might be split between stuff
>> >> + * done by QEMU itself and stuff handled by an external backend
>> >> + * (e.g. vhost). As a result some feature bits may be added or
>> >> + * masked from the backend.
>> >
>> > I'm not 100% sure what this is referring to. Transport features that
>> > are handled by QEMU and not the backend?
>>
>> Well there is the rub. While looking at the reset stuff it was
>> postulated a device could support reset even if vhost part couldn't.
>
> reset here referring to per-ring reset? It's possible with enough work
> - you would save ring state for each ring, reset all of vhost, then
> restore all but the one ring that needs to be reset.
Does anything work like this at the moment or is it a fair assumption
that the feature set of a vhost/vhost-user backend will be the maximum
set of features the guest can select from?
>
>> If
>> that is not true maybe we should drop this because host_features should
>> have everything we need?
>>
>> >
>> >> + */
>> >> uint64_t host_features;
>> >> + uint64_t guest_features;
>> >> uint64_t backend_features;
>> >> +
>> >> size_t config_len;
>> >> void *config;
>> >> uint16_t config_vector;
>> >> --
>> >> 2.34.1
>> >>
>>
>>
>> --
>> Alex Bennée
--
Alex Bennée
