From mboxrd@z Thu Jan 1 00:00:00 1970 From: John Garry Subject: Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands Date: Tue, 7 Apr 2020 12:54:29 +0100 Message-ID: <9c6ced82-b3f1-9724-b85e-d58827f1a4a4@huawei.com> References: <1583857550-12049-1-git-send-email-john.garry@huawei.com> <1583857550-12049-3-git-send-email-john.garry@huawei.com> <20200310183243.GA14549@infradead.org> <79cf4341-f2a2-dcc9-be0d-2efc6e83028a@huawei.com> <20200311062228.GA13522@infradead.org> Mime-Version: 1.0 Content-Type: text/plain; charset="utf-8"; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: Content-Language: en-US Sender: linux-kernel-owner@vger.kernel.org To: Hannes Reinecke , Christoph Hellwig Cc: axboe@kernel.dk, jejb@linux.ibm.com, martin.petersen@oracle.com, ming.lei@redhat.com, bvanassche@acm.org, linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, linux-scsi@vger.kernel.org, virtualization@lists.linux-foundation.org, esc.storagedev@microsemi.com, chenxiang66@hisilicon.com, Hannes Reinecke List-Id: virtualization@lists.linuxfoundation.org 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 >>>>> >>>>> 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 , > 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