On Thu, 2015-09-10 at 15:38 +0100, Stefan Hajnoczi wrote:

On Thu, Sep 10, 2015 at 6:48 AM, Ming Lin [off-list ref] wrote:

quoted

These 2 patches added virtio-nvme to kernel and qemu,
basically modified from virtio-blk and nvme code.

As title said, request for your comments.

Play it in Qemu with:
-drive file=disk.img,format=raw,if=none,id=D22 \
-device virtio-nvme-pci,drive=D22,serial=1234,num_queues=4

The goal is to have a full NVMe stack from VM guest(virtio-nvme)
to host(vhost_nvme) to LIO NVMe-over-fabrics target.

Why is a virtio-nvme guest device needed?  I guess there must either
be NVMe-only features that you want to pass through, or you think the
performance will be significantly better than virtio-blk/virtio-scsi?

It simply passes through NVMe commands.

Right now performance is poor. Performance tunning is on my todo list.
It should be as good as virtio-blk/virtio-scsi.

At first glance it seems like the virtio_nvme guest driver is just
another block driver like virtio_blk, so I'm not clear why a
virtio-nvme device makes sense.

I think the future "LIO NVMe target" only speaks NVMe protocol.

Nick(CCed), could you correct me if I'm wrong?

For SCSI stack, we have:
virtio-scsi(guest)
tcm_vhost(or vhost_scsi, host)
LIO-scsi-target

For NVMe stack, we'll have similar components:
virtio-nvme(guest)
vhost_nvme(host)
LIO-NVMe-target

quoted

Now there are lots of duplicated code with linux/nvme-core.c and qemu/nvme.c.
The ideal result is to have a multi level NVMe stack(similar as SCSI).
So we can re-use the nvme code, for example

                        .-------------------------.
                        | NVMe device register    |
  Upper level           | NVMe protocol process   |
                        |                         |
                        '-------------------------'



              .-----------.    .-----------.    .------------------.
 Lower level  |   PCIe    |    | VIRTIO    |    |NVMe over Fabrics |
              |           |    |           |    |initiator         |
              '-----------'    '-----------'    '------------------'

You mentioned LIO and SCSI.  How will NVMe over Fabrics be integrated
into LIO?  If it is mapped to SCSI then using virtio_scsi in the guest
and tcm_vhost should work.

I think it's not mapped to SCSI.

Nick, would you share more here?

Please also post virtio draft specifications documenting the virtio device.

I'll do this later.

Stefan