Libvirt NVME support

[Michal Prívozník]

On 11/23/20 3:03 PM, Daniel P. Berrangé wrote:
> On Wed, Nov 18, 2020 at 11:24:30AM +0100, Peter Krempa wrote:
>> On Wed, Nov 18, 2020 at 09:57:14 +0000, Thanos Makatos wrote:
>>>> As a separate question, is there any performance benefit of emulating a
>>>> NVMe controller compared to e.g. virtio-scsi?
>>>
>>> We haven't measured that yet; I would expect it to be slight faster and/or more
>>> CPU efficient but wouldn't be surprised if it isn't. The main benefit of using
>>> NVMe is that we don't have to install VirtIO drivers in the guest.
>>
>> Okay, I'm not sold on the drivers bit but that is definitely not a
>> problem in regards of adding support for emulating NVME controllers to
>> libvirt.
>>
>> As a starting point a trivial way to model this in the XML will be:
>>
>>      <controller type='nvme' index='1' model='nvme'>
>>
>> And then add the storage into it as:
>>
>>      <disk type='file' device='disk'>
>>        <source dev='/Host/QEMUGuest1.qcow2'/>
>>        <target dev='sda' bus='nvme'/>
>>        <address type='drive' controller='1' bus='0' target='0' unit='0'/>
>>      </disk>
>>
>>      <disk type='file' device='disk'>
>>        <source dev='/Host/QEMUGuest2.qcow2'/>
>>        <target dev='sdb' bus='nvme'/>
>>        <address type='drive' controller='1' bus='0' target='0' unit='1'/>
>>      </disk>
>>
>> The 'drive' address here maps the disk to the controller. This example
>> uses unit= as the way to specify the namespace ID. Both 'bus' and 'target'
>> must be 0.
> 
> FWIW, I think that our overloeading of type=drive for FDC, IDE, and SCSI
> was a mistake in retrospect. We should have had type=fdc, type=ide, type=scsi,
> since each uses a different subset of the attributes.
> 
> Lets not continue this mistake with NVME - create a type=nvme address
> type.

Don't NVMes live on a PCI(e) bus? Can't we just threat NVMes as PCI 
devices? Or are we targeting sata too? Bcause we also have that type of 
address.

Michal