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Re: [libvirt-users] [libvirt] Add support for vhost-user-scsi-pci/vhost-user-blk-pci



Sorry for the late reply, and thanks Jano for pointing out elsewhere that this didn't receive a response.

On 8/12/19 5:56 AM, Li Feng wrote:
Hi Guys,

And I want to add the vhost-user-scsi-pci/vhost-user-blk-pci support
for libvirt.

The usage in qemu like this:

Vhost-SCSI
-chardev socket,id=char0,path=/var/tmp/vhost.0
-device vhost-user-scsi-pci,id=scsi0,chardev=char0
Vhost-BLK
-chardev socket,id=char1,path=/var/tmp/vhost.1
-device vhost-user-blk-pci,id=blk0,chardev=char1


Indeed that matches what I see for the qemu commits too:

https://git.qemu.org/?p=qemu.git;a=commit;h=00343e4b54b
https://git.qemu.org/?p=qemu.git;a=commit;h=f12c1ebddf7

What type should I add for libvirt.
Type1:
     <hostdev mode='subsystem' type='vhost-user'>
         <source protocol='vhost-user-scsi' path='/tmp/vhost-scsi.sock'></source>
         <alias name="vhost-user-scsi-disk1"/>
     </hostdev>


Type2:

     <disk type='network' device='disk'>
       <driver name='qemu' type='raw' cache='none' io='native'/>
       <source protocol='vhost-user' path='/tmp/vhost-scsi.sock'>
       </source>
       <target dev='sdb' bus='vhost-user-scsi'/>
       <boot order='3'/>
       <alias name='scsi0-0-0-1'/>
       <address type='drive' controller='0' bus='0' target='0' unit='1'/>
     </disk>


     <disk type='network' device='disk'>
       <driver name='qemu' type='raw' cache='none' io='native'/>
       <source protocol='vhost-user' path='/tmp/vhost-blk.sock'>
       </source>
       <target dev='vda' bus='vhost-user-blk'/>
       <boot order='1'/>
       <alias name='virtio-disk0'/>
       <address type='pci' domain='0x0000' bus='0x00' slot='0x07'
function='0x0'/>
     </disk>


I think wiring this into <disk> makes more sense. <hostdev> is really an abstraction for assigning a (typically) physical host device to the VM, so it handles things like hiding a PCI device from the host, and passing that exact device to the VM.

In the vhost-user-scsi/blk case, the host device is just a special process running on the other side of a socket, and the device represented to the guest is a typical virtio device. So to me it makes more sense as a <disk> with a <source> that points at that socket.

target bus=virtio vs bus=scsi is already used to distinguish between virtio-blk and virtio-scsi, so I think we can keep that bit as is, with the <address type=drive|pci> to match. We just need to differentiate between plain virtio and vhost-user

network devices already have vhostuser support:

  <interface type='vhostuser'>
    <source type='unix' path='/tmp/vhost1.sock' mode='server|client'/>
    <model type='virtio'/>
  </interface>

Internally that <source> is a virDomainChrSourceDefPtr which is our internal representation of a chardev. So I think something akin to this is the way to go. It will likely require updating a LOT of places in the code that check disk type= field, probably most places that care about whether type=NETWORK or type=!NETWORK will need to be mirrored for the new type.

<disk type='vhostuser' device='disk'>
  <source type='unix' path='/path/to/vhost-user-blk.sock' mode='client'/>
  <target dev='vda' bus='virtio'/>
</disk>

<disk type='vhostuser' device='disk'>
  <source type='unix' path='/path/to/vhost-user-scsi.sock' mode='client'/>
  <target dev='sda' bus='scsi'/>
</disk>

- Cole


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