Re: Pe: [PATCH v5 1/3] virtio-scsi: first version

From: Paolo Bonzini
Date: Tue Feb 07 2012 - 04:55:08 EST

On 02/06/2012 10:51 AM, Christian Hoff wrote:
Hello Paolo,

first let me say that your patch is working fine on my local clone of the
qemu repository.

Let me ask just one question about the format of the data being
transmitted over the virtqueue.

Paolo Bonzini wrote:
+ cmd->req.cmd = (struct virtio_scsi_cmd_req){
+ .lun[0] = 1,
+ .lun[1] = sc->device->id,
+ .lun[2] = (sc->device->lun>> 8) | 0x40,
+ .lun[3] = sc->device->lun& 0xff,
+ [...]
+ };

Can't we have seperate fields for the SCSI target ID and the LUN number
here? Putting all this into a single field seems confusing. The following
line of code (sc->device->lun>> 8) | 0x40 essentially means that LUN
numbers will be limited to 8+6 Bits=14 Bits for no obvious reason that I
can see. Maybe we could just split the LUN field up into two uint32 fields
for target ID and LUN number?

The 14-bit limitation can be lifted. SAM defines a 24-bit LUN format too, but I've never seen it used in practice.

Also, lun[1] = sc->device->id means that only 255 SCSI target IDs will be
supported. Think about bigger usage scenarios, such as FCP networks with
several hundred HBAs in the net. If you want to have the target ID<->HBA
mapping the same as on the guest as on the host, then 255 virtual target
IDs could be a limit.

I think you would hit other scalability limitations well before that. I plan to give each target its own MSI-X interrupt, but there is no infinite supplies of those either.

VMware only supports 15 targets and 255 LUNs per host, by comparison. I think 255 targets and 16383 LUNs is already several times more than is actually needed. But in any case, we could still use the fixed "1" byte to go beyond 255 targets.

Sorry for coming up so late with these suggestions. I hope there is still
enough time left to discuss and address these problems.

Sure. :) I hope the above answer is satisfactory, though.

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