Re: [Qemu-devel] [PATCH v4 2/4] vhost-user-blk: introduce a new vhost-user-blk host device

[Stefan Hajnoczi]

On Tue, Oct 24, 2017 at 12:44:30AM +0000, Liu, Changpeng wrote:
> 
> 
> > -----Original Message-----
> > From: Stefan Hajnoczi [mailto:address@hidden
> > Sent: Tuesday, October 24, 2017 1:12 AM
> > To: Liu, Changpeng <address@hidden>
> > Cc: address@hidden; address@hidden; address@hidden;
> > address@hidden; address@hidden; Harris, James R
> > <address@hidden>
> > Subject: Re: [PATCH v4 2/4] vhost-user-blk: introduce a new vhost-user-blk 
> > host
> > device
> > 
> > On Mon, Oct 23, 2017 at 04:26:36AM +0000, Liu, Changpeng wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Stefan Hajnoczi [mailto:address@hidden
> > > > Sent: Friday, October 20, 2017 5:55 PM
> > > > To: Liu, Changpeng <address@hidden>
> > > > Cc: address@hidden; address@hidden; address@hidden;
> > > > address@hidden; address@hidden; Harris, James R
> > > > <address@hidden>
> > > > Subject: Re: [PATCH v4 2/4] vhost-user-blk: introduce a new 
> > > > vhost-user-blk
> > host
> > > > device
> > > >
> > > > On Fri, Oct 20, 2017 at 01:47:58AM +0000, Liu, Changpeng wrote:
> > > > > > > +static Property vhost_user_blk_properties[] = {
> > > > > > > +    DEFINE_PROP_CHR("chardev", VHostUserBlk, chardev),
> > > > > > > +    DEFINE_PROP_UINT16("num_queues", VHostUserBlk, num_queues,
> > 1),
> > > > > > > +    DEFINE_PROP_UINT32("queue_size", VHostUserBlk, queue_size, 
> > > > > > > 128),
> > > > > > > +    DEFINE_PROP_BIT64("f_size_max", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_SIZE_MAX, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_sizemax", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_SIZE_MAX, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_segmax", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_SEG_MAX, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_geometry", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_GEOMETRY, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_readonly", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_RO, false),
> > > > > > > +    DEFINE_PROP_BIT64("f_blocksize", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_BLK_SIZE, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_topology", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_TOPOLOGY, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_multiqueue", VHostUserBlk, 
> > > > > > > host_features,
> > > > > > > +                      VIRTIO_BLK_F_MQ, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_flush", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_FLUSH, true),
> > > > > > > +    DEFINE_PROP_BIT64("f_barrier", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_BARRIER, false),
> > > > > > > +    DEFINE_PROP_BIT64("f_scsi", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_SCSI, false),
> > > > > > > +    DEFINE_PROP_BIT64("f_wce", VHostUserBlk, host_features,
> > > > > > > +                      VIRTIO_BLK_F_WCE, false),
> > > > > >
> > > > > > Please explain how feature negotation works.  The vhost-user slave
> > > > > > advertises support features in the return value from
> > > > > > VHOST_USER_GET_FEATURES.  How does this additional feature mask
> > work
> > > > and
> > > > > > why is it useful?
> > > > > According to Paolo's previous comments, VIRTIO_BLK_F_WCE/
> > > > VIRTIO_BLK_F_SCSI/ VIRTIO_BLK_F_BARRIER
> > > > > should be removed. Here I added all the feature flags just want to 
> > > > > avoid
> > the
> > > > case that vhost-user slave target
> > > > > can support but Qemu vhost block driver cannot support it.
> > > >
> > > > Please explain a bit more how these options can be used.
> > > >
> > > > When I looked at the vhost code it seemed like the vhost slave can
> > > > report any feature bits it wishes (even things QEMU doesn't know about).
> > > > What is the purpose of override some of the feature bits on the QEMU
> > > > command-line?
> > > Hi Stefan,
> > > Here I added a switch which can override vhost-slave's feature bits, for
> > example, vhost-slave reported `VIRTIO_BLK_F_RO`,
> > > but Qemu vhost-master can disable it through command line when started the
> > Qemu. Users don't need to change any
> > > vhost-slave's code to disable this feature, and this is also aligned with 
> > > vhost-
> > scsi and vhost-net's implementation.
> > 
> > You said vhost-master can disable features but the code doesn't seem to
> > work that way:
> > 
> > +    /* Turn on pre-defined features */
> > +    features |= s->host_features;
> User can append parameter when started Qemu: e.g: f_readonly=false to disable 
> it.

No, f_readonly=false has no effect if the vhost slave replies with f_readonly
to the VHOST_USER_GET_FEATURES message.

Take a look at the hw/virtio/vhost.c code:

  uint64_t vhost_get_features(struct vhost_dev *hdev, const int *feature_bits,
                              uint64_t features)
  {
      const int *bit = feature_bits;
      while (*bit != VHOST_INVALID_FEATURE_BIT) {
          uint64_t bit_mask = (1ULL << *bit);
          if (!(hdev->features & bit_mask)) {
              features &= ~bit_mask;
          }
          bit++;
      }
      return features;
  }

If f_readonly=false then features |= s->host_features will not contain
f_readonly but it doesn't matter because if hdev->features (this value
comes from the vhost slave) has f_readonly we will not clear the bit!

So again, what is the purpose of the f_* properties?  I don't understand
the semantics, it seems useless.