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



On 10/14/19 3:12 AM, Li Feng wrote:
> Hi Cole & Michal,
> 
> I'm sorry for my late response, I just end my journey today.
> Thank your response, your suggestion is very helpful to me.
> 
> I have added Michal in this mail, Michal helps me review my initial patchset.
> (https://www.spinics.net/linux/fedora/libvir/msg191339.html)
> 

Whoops I missed that posting, I didn't realize you had sent patches!

> All concern about this feature is the XML design.
> My original XML design exposes more details of Qemu.
> 
>      <vhost-user-blk-pci type='unix'>
>          <source type='bind' path='/tmp/vhost-blk.sock'>
>              <reconnect enabled='yes' timeout='5' />
>          </source>
>          <queue num='4'/>
>      </vhost-user-blk-pci>
> 
> As Cole's suggestion, the better design with all vhost-user-scsi/blk
> features would like this:
> 
> vhost-user-blk:
> 
> <disk type='vhostuser' device='disk'>
>     <source type='unix' path='/path/to/vhost-user-blk.sock' mode='client'>
>         <reconnect enabled='yes' timeout='5' />
>      </source>
>     <target dev='vda' bus='virtio'/>
>     <queue num='4'/>
>      <address type='pci' domain='0x0000' bus='0x00' slot='0x07' function='0x0'/>
> </disk>
> 
> vhost-user-scsi:
> 
> <disk type='vhostuser' device='disk'>
>     <source type='unix' path='/path/to/vhost-user-scsi.sock' mode='client'>
>         <reconnect enabled='yes' timeout='5' />
>     </source>
>     <target dev='sda' bus='scsi'/>
>     <queue num='4'/>
> </disk>
> 

I think my SCSI idea is wrong, sorry. vhost-user-scsi is for passing a
scsi host adapter to the VM, correct? If so, then it's not really a
<disk>, and so using the existing vhost-scsi support in <hostdev> is
probably better. <hostdev> could possible be used for vhost-user-blk as well

Can you provide some examples of full qemu command lines using
vhost-user-blk and vhost-user-scsi? Just linking to examples else where
is fine, but I'm wondering if there's more context

Internally we already have an abstraction for vhost-scsi:

    <hostdev mode='subsystem' type='scsi_host'>
      <source protocol='vhost' wwpn='XXX'/>
    </hostdev>


The obvious extension would be

    <hostdev mode='subsystem' type='scsi_host'>
      <source protocol='vhostuser' type='unix'
path='/path/to/vhost-user-scsi.sock' mode='client'/>
    </hostdev>

Internally implementing this will be weird. The <source> parameters are
only dictated by the hostdev type= field, but in this case they would be
dictated by the <source protocol=> field, and we would want to reuse the
internal chardev abstraction.

vhost-user-blk could be implemented similarly, but with type='storage'
which is the way we pass through block devices to LXC guests, but it
isn't currently supported in the qemu driver.

I dunno. Maybe Michal or danpb can provide guidance


> Conclusion:
> 1. Add new type(vhostuser) for disk label;
> 2. Add queue sub-label for disk to support multiqueue(<queue
> num='4'/>) or reusing the driver label
> (<driver name='vhostuser' queues='4'), which one is better?
> Qemu support multiqueue like this:
> -device vhost-user-scsi-pci,id=scsi0,chardev=spdk_vhost_scsi0,num_queues=4
> -device vhost-user-blk-pci,chardev=spdk_vhost_blk0,num-queues=4
> 

num-queues is already supported by libvirt for both <disk> and <hostdev>
with <driver queues=X/>, so whether we use <disk> or <hostdev> you won't
need to add any new XML here.

> Another question:
> When qemu is connecting to a vhost-user-scsi controller[1],  there may
> exist multiple LUNs under one target,
> then one disklabel(<disk/>) will represent multiple SCSI LUNs,
> the 'dev' property(<target dev='sda' bus='scsi'/>) will be ignored, right?
> In other words, for vhost-user-scsi disk, it more likes a controller,
> maybe the controller label is suitable.
> 

Yes you are right, and this was my understanding. But then its not
really a <controller> in libvirt's sense because we can't attach
emulated devices to it, so it's more a fit for the existing <hostdev>
vhost-user support. Unfortunately it's not really a clean fit anywhere,
there will have to be some kind of compromise. And I'm not sure if
<disk> or <hostdev> is right for vhost-user-blk, but hopefully others
have more clear opinions.

Thanks,
Cole

> I look forward to hearing from you as soon as possible.
> 
> [1]: https://spdk.io/doc/vhost.html
> 
> Feng Li
> 
> Cole Robinson <crobinso redhat com> 于2019年10月10日周四 上午6:48写道:
>>
>> 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
> 


- Cole


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