Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands

[John Garry <john.garry@huawei.com>]

On 06/04/2020 10:05, Hannes Reinecke wrote:
> On 3/11/20 7:22 AM, Christoph Hellwig wrote:
>> On Tue, Mar 10, 2020 at 09:08:56PM +0000, John Garry wrote:
>>> On 10/03/2020 18:32, Christoph Hellwig wrote:
>>>> On Wed, Mar 11, 2020 at 12:25:28AM +0800, John Garry wrote:
>>>>> From: Hannes Reinecke <hare@suse.com>
>>>>>
>>>>> Allocate a separate 'reserved_cmd_q' for sending reserved commands.
>>>>
>>>> Why?  Reserved command specifically are not in any way tied to queues.
>>>> .
>>>>
>>>
>>> So the v1 series used a combination of the sdev queue and the per-host
>>> reserved_cmd_q. Back then you questioned using the sdev queue for virtio
>>> scsi, and the unconfirmed conclusion was to use a common per-host q. This is
>>> the best link I can find now:
>>>
>>> https://www.mail-archive.com/linux-scsi@vger.kernel.org/msg83177.html
>>
>> That was just a question on why virtio uses the per-device tags, which
>> didn't look like it made any sense.  What I'm worried about here is
>> mixing up the concept of reserved tags in the tagset, and queues to use
>> them.  Note that we already have the scsi_get_host_dev to allocate
>> a scsi_device and thus a request_queue for the host itself.  That seems
>> like the better interface to use a tag for a host wide command vs
>> introducing a parallel path.
>>
> Thinking about it some more, I don't think that scsi_get_host_dev() is
> the best way of handling it.
> Problem is that it'll create a new scsi_device with <hostno:this_id:0>,
> which will then show up via eg 'lsscsi'.

are you sure? Doesn't this function just allocate the sdev, but do 
nothing with it, like probing it?

I bludgeoned it in here for PoC:

https://github.com/hisilicon/kernel-dev/commit/ef0ae8540811e32776f64a5b42bd76cbed17ba47

And then still:

john@ubuntu:~$ lsscsi
[0:0:0:0] disk SEAGATE  ST2000NM0045  N004  /dev/sda
[0:0:1:0] disk SEAGATE  ST2000NM0045  N004  /dev/sdb
[0:0:2:0] disk ATASAMSUNG HM320JI  0_01  /dev/sdc
[0:0:3:0] disk SEAGATE  ST1000NM0023  0006  /dev/sdd
[0:0:4:0] enclosu HUAWEIExpander 12Gx16  128-
john@ubuntu:~$

Some proper plumbing would be needed, though.

> This would be okay if 'this_id' would have been defined by the driver;
> sadly, most drivers which are affected here do set 'this_id' to -1.
> So we wouldn't have a nice target ID to allocate the device from, let
> alone the problem that we would have to emulate a complete scsi device
> with all required minimal command support etc.
> And I'm not quite sure how well that would play with the exising SCSI
> host template; the device we'll be allocating would have basically
> nothing in common with the 'normal' SCSI devices.
> 
> What we could do, though, is to try it the other way round:
> Lift the request queue from scsi_get_host_dev() into the scsi host
> itself, so that scsi_get_host_dev() can use that queue, but we also
> would be able to use it without a SCSI device attached.

wouldn't that limit 1x scsi device per host, not that I know if any more 
would ever be required? But it does still seem better to use the request 
queue in the scsi device.

> 

cheers,
John