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

[Alex Bennée]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> 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?

Well I only came across it when I realised by rpmb implementation wasn't
sending config updates to the vhost-user daemon because it only
implemented set/get_config methods.

I think vhost-user-scsi suffers from this but I'm not able to test it as
it's not clear how to do it. The vhost-user-scsi daemon want to attach
to real SCSI devices rather than a file for the block device. I guess I
need to somehow add "fake" scsi nodes to my host system which SCSI
commands can be passed to?

When was the last time vhost-user-scsi was tested with anything?

>> 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 ...

But that would be a bug right? Certainly a mismatch if something really
does want to see config messages. Again RPMB needs this because it's the
vhost-user daemon that knows the size of the device.

>> 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.

No I think it's broken, we just didn't notice because as you say most
devices don't need to care.

>
>
>> 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


-- 
Alex Bennée