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@xxxxxxxx>
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@xxxxxxxxxxxxxxx/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
