Re: [PATCH v1 12/13] hw/virtio/vhost-user: don't suppress F_CONFIG when supported

[Michael S. Tsirkin]

On Mon, Mar 21, 2022 at 03:30:36PM +0000, Alex Bennée wrote:
> Previously we would silently suppress VHOST_USER_PROTOCOL_F_CONFIG
> during the protocol negotiation if the QEMU stub hadn't implemented
> the vhost_dev_config_notifier. However this isn't the only way we can
> handle config messages, the existing vdc->get/set_config can do this
> as well.


Could you give an example where the problem is encountered please?

> Lightly re-factor the code to check for both potential methods and
> instead of silently squashing the feature error out. It is unlikely
> that a vhost-user backend expecting to handle CONFIG messages will
> behave correctly if they never get sent.

Hmm but are you sure? Most devices work mostly fine without CONFIG
messages, there's a chance a backend set this flag just in case
without much thought ...

> Fixes: 1c3e5a2617 ("vhost-user: back SET/GET_CONFIG requests with a protocol 
> feature")

I'm not sure whether something is broken or this is a cleanup patch.
Fixes tag means "if you have 1c3e5a2617 you should pick this patch", so
cleanups don't need a fixes: tag.


> Cc: Maxime Coquelin <maxime.coquelin@redhat.com>
> Cc: Michael S. Tsirkin <mst@redhat.com>
> Cc: Stefan Hajnoczi <stefanha@redhat.com>
> Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
> 
> ---
>   - we can't check for get_config/set_config as the stack squashed vdev
>   - use vhost-user-state to transmit this
> ---
>  include/hw/virtio/vhost-user.h |  1 +
>  hw/scsi/vhost-user-scsi.c      |  1 +
>  hw/virtio/vhost-user.c         | 46 ++++++++++++++++++++++++----------
>  3 files changed, 35 insertions(+), 13 deletions(-)
> 
> diff --git a/include/hw/virtio/vhost-user.h b/include/hw/virtio/vhost-user.h
> index e44a41bb70..6e0e8a71a3 100644
> --- a/include/hw/virtio/vhost-user.h
> +++ b/include/hw/virtio/vhost-user.h
> @@ -22,6 +22,7 @@ typedef struct VhostUserState {
>      CharBackend *chr;
>      VhostUserHostNotifier notifier[VIRTIO_QUEUE_MAX];
>      int memory_slots;
> +    bool supports_config;
>  } VhostUserState;
>  
>  bool vhost_user_init(VhostUserState *user, CharBackend *chr, Error **errp);
> diff --git a/hw/scsi/vhost-user-scsi.c b/hw/scsi/vhost-user-scsi.c
> index 1b2f7eed98..9be21d07ee 100644
> --- a/hw/scsi/vhost-user-scsi.c
> +++ b/hw/scsi/vhost-user-scsi.c
> @@ -121,6 +121,7 @@ static void vhost_user_scsi_realize(DeviceState *dev, 
> Error **errp)
>      vsc->dev.backend_features = 0;
>      vqs = vsc->dev.vqs;
>  
> +    s->vhost_user.supports_config = true;
>      ret = vhost_dev_init(&vsc->dev, &s->vhost_user,
>                           VHOST_BACKEND_TYPE_USER, 0, errp);
>      if (ret < 0) {
> diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> index b27b8c56e2..6ce082861b 100644
> --- a/hw/virtio/vhost-user.c
> +++ b/hw/virtio/vhost-user.c
> @@ -1949,14 +1949,15 @@ static int 
> vhost_user_postcopy_notifier(NotifierWithReturn *notifier,
>  static int vhost_user_backend_init(struct vhost_dev *dev, void *opaque,
>                                     Error **errp)
>  {
> -    uint64_t features, protocol_features, ram_slots;
> +    uint64_t features, ram_slots;
>      struct vhost_user *u;
> +    VhostUserState *vus = (VhostUserState *) opaque;
>      int err;
>  
>      assert(dev->vhost_ops->backend_type == VHOST_BACKEND_TYPE_USER);
>  
>      u = g_new0(struct vhost_user, 1);
> -    u->user = opaque;
> +    u->user = vus;
>      u->dev = dev;
>      dev->opaque = u;
>  
> @@ -1967,6 +1968,10 @@ static int vhost_user_backend_init(struct vhost_dev 
> *dev, void *opaque,
>      }
>  
>      if (virtio_has_feature(features, VHOST_USER_F_PROTOCOL_FEATURES)) {
> +        bool supports_f_config = vus->supports_config ||
> +            (dev->config_ops && dev->config_ops->vhost_dev_config_notifier);
> +        uint64_t protocol_features;
> +
>          dev->backend_features |= 1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
>  
>          err = vhost_user_get_u64(dev, VHOST_USER_GET_PROTOCOL_FEATURES,
> @@ -1976,19 +1981,34 @@ static int vhost_user_backend_init(struct vhost_dev 
> *dev, void *opaque,
>              return -EPROTO;
>          }
>  
> -        dev->protocol_features =
> -            protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
> -
> -        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) 
> {
> -            /* Don't acknowledge CONFIG feature if device doesn't support it 
> */
> -            dev->protocol_features &= ~(1ULL << 
> VHOST_USER_PROTOCOL_F_CONFIG);
> -        } else if (!(protocol_features &
> -                    (1ULL << VHOST_USER_PROTOCOL_F_CONFIG))) {
> -            error_setg(errp, "Device expects VHOST_USER_PROTOCOL_F_CONFIG "
> -                       "but backend does not support it.");
> -            return -EINVAL;
> +        /*
> +         * We will use all the protocol features we support - although
> +         * we suppress F_CONFIG if we know QEMUs internal code can not 
> support
> +         * it.
> +         */
> +        protocol_features &= VHOST_USER_PROTOCOL_FEATURE_MASK;
> +
> +        if (supports_f_config) {
> +            if (!virtio_has_feature(protocol_features,
> +                                    VHOST_USER_PROTOCOL_F_CONFIG)) {
> +                error_setg(errp, "vhost-user device %s expecting "
> +                           "VHOST_USER_PROTOCOL_F_CONFIG but the vhost-user 
> backend does "
> +                           "not support it.", dev->vdev->name);
> +                return -EPROTO;
> +            }
> +        } else {
> +            if (virtio_has_feature(protocol_features,
> +                                   VHOST_USER_PROTOCOL_F_CONFIG)) {
> +                warn_reportf_err(*errp, "vhost-user backend supports "
> +                                 "VHOST_USER_PROTOCOL_F_CONFIG for "
> +                                 "device %s but QEMU does not.",
> +                                 dev->vdev->name);
> +                protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
> +            }
>          }
>  
> +        /* final set of protocol features */
> +        dev->protocol_features = protocol_features;
>          err = vhost_user_set_protocol_features(dev, dev->protocol_features);
>          if (err < 0) {
>              error_setg_errno(errp, EPROTO, "vhost_backend_init failed");
> -- 
> 2.30.2