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

[Stefan Hajnoczi <stefanha@gmail.com>]

On Mon, Oct 23, 2017 at 04:26:36AM +0000, Liu, Changpeng wrote:
> 
> 
> > -----Original Message-----
> > From: Stefan Hajnoczi [mailto:stefanha@gmail.com]
> > Sent: Friday, October 20, 2017 5:55 PM
> > To: Liu, Changpeng <changpeng.liu@intel.com>
> > Cc: qemu-devel@nongnu.org; pbonzini@redhat.com; mst@redhat.com;
> > marcandre.lureau@redhat.com; felipe@nutanix.com; Harris, James R
> > <james.r.harris@intel.com>
> > 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;

If the use case isn't clear please remove these properties for now.

Stefan