Re: [PATCH v2 4/6] vduse-blk: implements vduse-blk export

[Yongji Xie]

On Tue, Mar 15, 2022 at 7:08 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Tue, Feb 15, 2022 at 06:59:41PM +0800, Xie Yongji wrote:
> > This implements a VDUSE block backends based on
> > the libvduse library. We can use it to export the BDSs
> > for both VM and container (host) usage.
> >
> > The new command-line syntax is:
> >
> > $ qemu-storage-daemon \
> >     --blockdev file,node-name=drive0,filename=test.img \
> >     --export vduse-blk,node-name=drive0,id=vduse-export0,writable=on
> >
> > After the qemu-storage-daemon started, we need to use
> > the "vdpa" command to attach the device to vDPA bus:
> >
> > $ vdpa dev add name vduse-export0 mgmtdev vduse
>
> The per-QEMU export id is used as the global vdpa device name. If this
> becomes a problem in the future then a new --export
> vduse-blk,vdpa-dev-name= option can be added.
>

Yes.

> > +    case VIRTIO_BLK_T_GET_ID: {
> > +        size_t size = MIN(iov_size(&elem->in_sg[0], in_num),
> > +                          VIRTIO_BLK_ID_BYTES);
> > +        snprintf(elem->in_sg[0].iov_base, size, "%s", vblk_exp->export.id);
>
> Please use iov_from_buf(). The driver is allowed to submit as many
> in_sg[] elements as it wants and a compliant virtio-blk device
> implementation must support that.
>

Got it.

> Here is the VIRTIO specification section that covers message framing:
> https://docs.oasis-open.org/virtio/virtio/v1.1/cs01/virtio-v1.1-cs01.html#x1-280004
>
> > +    features = (1ULL << VIRTIO_F_IOMMU_PLATFORM) |
> > +               (1ULL << VIRTIO_F_VERSION_1) |
> > +               (1ULL << VIRTIO_RING_F_EVENT_IDX) |
> > +               (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
> > +               (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
> > +               (1ULL << VIRTIO_BLK_F_SIZE_MAX) |
> > +               (1ULL << VIRTIO_BLK_F_SEG_MAX) |
> > +               (1ULL << VIRTIO_BLK_F_TOPOLOGY) |
> > +               (1ULL << VIRTIO_BLK_F_BLK_SIZE);
>
> The VIRTIO_F_ and VIRTIO_RING_F_ feature bits report the capabilities of
> libvduse. They should probably be defined in libvduse. That way no
> changes to vduse-blk.c are required when libvduse changes:
>
>   features = LIBVDUSE_VIRTIO_FEATURES |
>              (1ULL << VIRTIO_BLK_F_SIZE_MAX) |
>              ...;

It's OK to define the VIRTIO_F_* feature bits in libvduse since daemon
might not want to disable it. But maybe not including VIRTIO_RING_F_*
feature bits since daemon might want to disable them in some cases.

Thanks,
Yongji