Re: [PATCH 2/5] vhost: involve device backends in feature negotiation

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, May 27, 2020 at 04:28:41PM +0200, Marc-André Lureau wrote:
> Hi Stefan
> 
> On Fri, May 22, 2020 at 7:18 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > Many vhost devices in QEMU currently do not involve the device backend
> > in feature negotiation. This seems fine at first glance for device types
> > without their own feature bits (virtio-net has many but other device
> > types have none).
> >
> > This overlooks the fact that QEMU's virtqueue implementation and the
> > device backend's implementation may support different features.  QEMU
> > must not report features to the guest that the the device backend
> > doesn't support.
> >
> > For example, QEMU supports VIRTIO 1.1 packed virtqueues while many
> > existing vhost device backends do not. When the user sets packed=on the
> > device backend breaks. This should have been handled gracefully by
> > feature negotiation instead.
> >
> > Introduce vhost_get_default_features() and update all vhost devices in
> > QEMU to involve the device backend in feature negotiation.
> >
> > This patch fixes the following error:
> >
> >   $ x86_64-softmmu/qemu-system-x86_64 \
> >       -drive if=virtio,file=test.img,format=raw \
> >       -chardev socket,path=/tmp/vhost-user-blk.sock,id=char0 \
> >       -device vhost-user-blk-pci,chardev=char0,packed=on \
> >       -object memory-backend-memfd,size=1G,share=on,id=ram0 \
> >       -M accel=kvm,memory-backend=ram0
> >   qemu-system-x86_64: Failed to set msg fds.
> >   qemu-system-x86_64: vhost VQ 0 ring restore failed: -1: Success (0)
> >
> > The vhost-user-blk backend failed as follows:
> >
> >   $ ./vhost-user-blk --socket-path=/tmp/vhost-user-blk.sock -b test2.img
> >   vu_panic: virtio: zero sized buffers are not allowed
> >   virtio-blk request missing headers
> >
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> >  include/hw/virtio/vhost.h        |  1 +
> >  include/hw/virtio/virtio-gpu.h   |  2 ++
> >  include/sysemu/cryptodev-vhost.h | 11 +++++++++++
> >  backends/cryptodev-vhost.c       | 19 +++++++++++++++++++
> >  hw/display/vhost-user-gpu.c      | 17 +++++++++++++++++
> >  hw/display/virtio-gpu-base.c     |  2 +-
> >  hw/input/vhost-user-input.c      |  9 +++++++++
> >  hw/virtio/vhost-user-fs.c        |  5 +++--
> >  hw/virtio/vhost-vsock.c          |  5 +++--
> >  hw/virtio/vhost.c                | 22 ++++++++++++++++++++++
> >  hw/virtio/virtio-crypto.c        |  3 ++-
> >  11 files changed, 90 insertions(+), 6 deletions(-)
> >
> > diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
> > index 085450c6f8..d2e54dd4a8 100644
> > --- a/include/hw/virtio/vhost.h
> > +++ b/include/hw/virtio/vhost.h
> > @@ -112,6 +112,7 @@ void vhost_virtqueue_mask(struct vhost_dev *hdev, VirtIODevice *vdev, int n,
> >                            bool mask);
> >  uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
> >                              uint64_t features);
> > +uint64_t vhost_get_default_features(struct vhost_dev *hdev, uint64_t features);
> >  void vhost_ack_features(struct vhost_dev *hdev, const int *feature_bits,
> >                          uint64_t features);
> >  bool vhost_has_free_slot(void);
> > diff --git a/include/hw/virtio/virtio-gpu.h b/include/hw/virtio/virtio-gpu.h
> > index 6dd57f2025..41d270d80e 100644
> > --- a/include/hw/virtio/virtio-gpu.h
> > +++ b/include/hw/virtio/virtio-gpu.h
> > @@ -192,6 +192,8 @@ bool virtio_gpu_base_device_realize(DeviceState *qdev,
> >  void virtio_gpu_base_reset(VirtIOGPUBase *g);
> >  void virtio_gpu_base_fill_display_info(VirtIOGPUBase *g,
> >                          struct virtio_gpu_resp_display_info *dpy_info);
> > +uint64_t virtio_gpu_base_get_features(VirtIODevice *vdev, uint64_t features,
> > +                                      Error **errp);
> >
> >  /* virtio-gpu.c */
> >  void virtio_gpu_ctrl_response(VirtIOGPU *g,
> > diff --git a/include/sysemu/cryptodev-vhost.h b/include/sysemu/cryptodev-vhost.h
> > index f42824fbde..e629446bfb 100644
> > --- a/include/sysemu/cryptodev-vhost.h
> > +++ b/include/sysemu/cryptodev-vhost.h
> > @@ -122,6 +122,17 @@ int cryptodev_vhost_start(VirtIODevice *dev, int total_queues);
> >   */
> >  void cryptodev_vhost_stop(VirtIODevice *dev, int total_queues);
> >
> > +/**
> > + * cryptodev_vhost_get_features:
> > + * @dev: the virtio crypto object
> > + * @requested_features: the features being offered
> > + *
> > + * Returns: the requested features bits that are supported by the vhost device,
> > + * or the original request feature bits if vhost is disabled
> > + *
> > + */
> > +uint64_t cryptodev_vhost_get_features(VirtIODevice *dev, uint64_t features);
> > +
> >  /**
> >   * cryptodev_vhost_virtqueue_mask:
> >   * @dev: the virtio crypto object
> > diff --git a/backends/cryptodev-vhost.c b/backends/cryptodev-vhost.c
> > index 8337c9a495..5f5a4fda7b 100644
> > --- a/backends/cryptodev-vhost.c
> > +++ b/backends/cryptodev-vhost.c
> > @@ -266,6 +266,20 @@ void cryptodev_vhost_stop(VirtIODevice *dev, int total_queues)
> >      assert(r >= 0);
> >  }
> >
> > +uint64_t cryptodev_vhost_get_features(VirtIODevice *dev, uint64_t features)
> > +{
> > +    VirtIOCrypto *vcrypto = VIRTIO_CRYPTO(dev);
> > +    CryptoDevBackend *b = vcrypto->cryptodev;
> > +    CryptoDevBackendClient *cc = b->conf.peers.ccs[0];
> > +    CryptoDevBackendVhost *vhost_crypto = cryptodev_get_vhost(cc, b, 0);
> > +
> > +    if (!vhost_crypto) {
> > +        return features; /* vhost disabled */
> > +    }
> > +
> > +    return vhost_get_default_features(&vhost_crypto->dev, features);
> > +}
> > +
> >  void cryptodev_vhost_virtqueue_mask(VirtIODevice *dev,
> >                                             int queue,
> >                                             int idx, bool mask)
> > @@ -333,6 +347,11 @@ void cryptodev_vhost_stop(VirtIODevice *dev, int total_queues)
> >  {
> >  }
> >
> > +uint64_t cryptodev_vhost_get_features(VirtIODevice *dev, uint64_t features)
> > +{
> > +    return features;
> > +}
> > +
> >  void cryptodev_vhost_virtqueue_mask(VirtIODevice *dev,
> >                                      int queue,
> >                                      int idx, bool mask)
> > diff --git a/hw/display/vhost-user-gpu.c b/hw/display/vhost-user-gpu.c
> > index 4cdaee1bde..e483df2a9e 100644
> > --- a/hw/display/vhost-user-gpu.c
> > +++ b/hw/display/vhost-user-gpu.c
> > @@ -466,6 +466,22 @@ vhost_user_gpu_set_config(VirtIODevice *vdev,
> >      }
> >  }
> >
> > +static uint64_t
> > +vhost_user_gpu_get_features(VirtIODevice *vdev, uint64_t features,
> > +                            Error **errp)
> > +{
> > +    VhostUserGPU *g = VHOST_USER_GPU(vdev);
> > +    Error *local_err = NULL;
> > +
> > +    features = virtio_gpu_base_get_features(vdev, features, &local_err);
> > +    if (local_err) {
> > +        error_propagate(errp, local_err);
> > +        return 0;
> > +    }
> > +
> > +    return vhost_get_default_features(&g->vhost->dev, features);
> > +}
> > +
> >  static void
> >  vhost_user_gpu_set_status(VirtIODevice *vdev, uint8_t val)
> >  {
> > @@ -582,6 +598,7 @@ vhost_user_gpu_class_init(ObjectClass *klass, void *data)
> >
> >      vdc->realize = vhost_user_gpu_device_realize;
> >      vdc->reset = vhost_user_gpu_reset;
> > +    vdc->get_features = vhost_user_gpu_get_features;
> >      vdc->set_status   = vhost_user_gpu_set_status;
> >      vdc->guest_notifier_mask = vhost_user_gpu_guest_notifier_mask;
> >      vdc->guest_notifier_pending = vhost_user_gpu_guest_notifier_pending;
> > diff --git a/hw/display/virtio-gpu-base.c b/hw/display/virtio-gpu-base.c
> > index c159351be3..05d1ff2db2 100644
> > --- a/hw/display/virtio-gpu-base.c
> > +++ b/hw/display/virtio-gpu-base.c
> > @@ -176,7 +176,7 @@ virtio_gpu_base_device_realize(DeviceState *qdev,
> >      return true;
> >  }
> >
> > -static uint64_t
> > +uint64_t
> >  virtio_gpu_base_get_features(VirtIODevice *vdev, uint64_t features,
> >                               Error **errp)
> >  {
> > diff --git a/hw/input/vhost-user-input.c b/hw/input/vhost-user-input.c
> > index 63984a8ba7..1371fb32cc 100644
> > --- a/hw/input/vhost-user-input.c
> > +++ b/hw/input/vhost-user-input.c
> > @@ -45,6 +45,14 @@ static void vhost_input_change_active(VirtIOInput *vinput)
> >      }
> >  }
> >
> > +static uint64_t vhost_input_get_features(VirtIODevice *vdev, uint64_t features,
> > +                                         Error **errp)
> > +{
> > +    VHostUserInput *vhi = VHOST_USER_INPUT(vdev);
> > +
> > +    return vhost_get_default_features(&vhi->vhost->dev, features);
> > +}
> > +
> >  static void vhost_input_get_config(VirtIODevice *vdev, uint8_t *config_data)
> >  {
> >      VirtIOInput *vinput = VIRTIO_INPUT(vdev);
> > @@ -89,6 +97,7 @@ static void vhost_input_class_init(ObjectClass *klass, void *data)
> >      DeviceClass *dc = DEVICE_CLASS(klass);
> >
> >      dc->vmsd = &vmstate_vhost_input;
> > +    vdc->get_features = vhost_input_get_features;
> >      vdc->get_config = vhost_input_get_config;
> >      vdc->set_config = vhost_input_set_config;
> >      vic->realize = vhost_input_realize;
> > diff --git a/hw/virtio/vhost-user-fs.c b/hw/virtio/vhost-user-fs.c
> > index 1bc5d03a00..56015ca3d4 100644
> > --- a/hw/virtio/vhost-user-fs.c
> > +++ b/hw/virtio/vhost-user-fs.c
> > @@ -130,8 +130,9 @@ static uint64_t vuf_get_features(VirtIODevice *vdev,
> >                                        uint64_t requested_features,
> >                                        Error **errp)
> >  {
> > -    /* No feature bits used yet */
> > -    return requested_features;
> > +    VHostUserFS *fs = VHOST_USER_FS(vdev);
> > +
> > +    return vhost_get_default_features(&fs->vhost_dev, requested_features);
> >  }
> >
> >  static void vuf_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > diff --git a/hw/virtio/vhost-vsock.c b/hw/virtio/vhost-vsock.c
> > index 4a228f5168..7276587be6 100644
> > --- a/hw/virtio/vhost-vsock.c
> > +++ b/hw/virtio/vhost-vsock.c
> > @@ -180,8 +180,9 @@ static uint64_t vhost_vsock_get_features(VirtIODevice *vdev,
> >                                           uint64_t requested_features,
> >                                           Error **errp)
> >  {
> > -    /* No feature bits used yet */
> > -    return requested_features;
> > +    VHostVSock *vsock = VHOST_VSOCK(vdev);
> > +
> > +    return vhost_get_default_features(&vsock->vhost_dev, requested_features);
> >  }
> >
> >  static void vhost_vsock_handle_output(VirtIODevice *vdev, VirtQueue *vq)
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index aff98a0ede..f8a144dcd0 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -48,6 +48,23 @@ static unsigned int used_memslots;
> >  static QLIST_HEAD(, vhost_dev) vhost_devices =
> >      QLIST_HEAD_INITIALIZER(vhost_devices);
> >
> > +/*
> > + * Feature bits that device backends must explicitly report. Feature bits not
> > + * listed here maybe set by QEMU without checking with the device backend.
> > + * Ideally all feature bits would be listed here but existing vhost device
> > + * implementations do not explicitly report bits like VIRTIO_F_VERSION_1, so we
> > + * can only assume they are supported.
> > + *
> > + * New feature bits added to the VIRTIO spec should usually be included here
> > + * so that existing vhost device backends that do not support them yet continue
> > + * to work.
> > + */
> > +static const int vhost_default_feature_bits[] = {
> > +    VIRTIO_F_IOMMU_PLATFORM,
> > +    VIRTIO_F_RING_PACKED,
> 
> So effectively, we don't care about backend features except for those
> 2 bits, and an extra F_LOG_ALL check in vhost.c, right?
> 
> It's not entirely clear to me how the feature flags of device and
> backend should combine tbh.

I've always found this confusing too. Maybe a vhost-user spec
clarification would help improve the situation at least for future
vhost-user backend implementors.

libvhost-user only advertises VHOST_F_LOG_ALL and
VHOST_USER_F_PROTOCOL_FEATURES. Everything else is left up to the device
backend implementation. This is a problem because libvhost-user handles
virtqueues and needs to be aware of VIRTIO_RING_F_EVENT_IDX, etc. I'll
send a separate patch to advertise the necessary feature bits in
libvhost-user.

QEMU calculates the VIRTIO device's feature bit masks as follows:

1. -device vhost-foo initializes the device features with whatever QEMU
   supports. Either the QEMU vhost-foo code itself sets features or the
   user overrides them on the command-line.

2. Some vhost devices in QEMU call vhost_get_features() to mask out
   features missing from the VHOST_USER_GET_FEATURES response. This is
   how the vhost-user backend participates in feature negotiation.

This patch series changes moves us one step in the direction of always
involving the vhost-user backend in feature negotiation. It's only a
step because to do so fully would break existing vhost-user backends
that don't advertise features properly :(.

Stefan

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