Target-devel archive on lore.kernel.org
 help / color / Atom feed
From: Jason Wang <jasowang@redhat.com>
To: Mike Christie <michael.christie@oracle.com>,
	martin.petersen@oracle.com, linux-scsi@vger.kernel.org,
	target-devel@vger.kernel.org, mst@redhat.com,
	pbonzini@redhat.com, stefanha@redhat.com,
	virtualization@lists.linux-foundation.org
Subject: Re: [PATCH 07/17] vhost scsi: support delayed IO vq creation
Date: Wed, 28 Oct 2020 01:55:15 +0000
Message-ID: <adcaeb64-b1da-2195-6656-ecc19f12e5ed@redhat.com> (raw)
In-Reply-To: <49c2fc29-348c-06db-4823-392f7476d318@oracle.com>


On 2020/10/27 下午1:47, Mike Christie wrote:
> On 10/25/20 10:51 PM, Jason Wang wrote:
>>
>> On 2020/10/22 上午8:34, Mike Christie wrote:
>>> Each vhost-scsi device will need a evt and ctl queue, but the number
>>> of IO queues depends on whatever the user has configured in userspace.
>>> This patch has vhost-scsi create the evt, ctl and one IO vq at device
>>> open time. We then create the other IO vqs when userspace starts to
>>> set them up. We still waste some mem on the vq and scsi vq structs,
>>> but we don't waste mem on iovec related arrays and for later patches
>>> we know which queues are used by the dev->nvqs value.
>>>
>>> Signed-off-by: Mike Christie <michael.christie@oracle.com>
>>> ---
>>>   drivers/vhost/scsi.c | 19 +++++++++++++++----
>>>   1 file changed, 15 insertions(+), 4 deletions(-)
>>
>>
>> Not familiar with SCSI. But I wonder if it could behave like vhost-net.
>>
>> E.g userspace should known the number of virtqueues so it can just 
>> open and close multiple vhost-scsi file descriptors.
>>
>
> One hiccup I'm hitting is that we might end up creating about 3x more 
> vqs than we need. The problem is that for scsi each vhost device has:
>
> vq=0: special control vq
> vq=1: event vq
> vq=2 and above: SCSI CMD/IO vqs. We want to create N of these.
>
> Today we do:
>
> Uerspace does open(/dev/vhost-scsi)
>         vhost_dev_init(create 128 vqs and then later we setup and use 
> N of them);
>
> Qemu does ioctl(VHOST_SET_OWNER)
>         vhost_dev_set_owner()
>
> For N vqs userspace does:
>         // virtqueue setup related ioctls
>
> Qemu does ioctl(VHOST_SCSI_SET_ENDPOINT)
>         - match LIO/target port to vhost_dev
>
>
> So we could change that to:
>
> For N IO vqs userspace does
>         open(/dev/vhost-scsi)
>                 vhost_dev_init(create IO, evt, and ctl);
>
> for N IO vqs Qemu does:
>         ioctl(VHOST_SET_OWNER)
>                 vhost_dev_set_owner()
>
> for N IO vqs Qemu does:
>         // virtqueue setup related ioctls
>
> for N IO vqs Qemu does:
>         ioctl(VHOST_SCSI_SET_ENDPOINT)
>                 - match LIO/target port to vhost_dev and assemble the 
> multiple vhost_dev device.
>
> The problem is that we have to setup some of the evt/ctl specific 
> parts at open() time when vhost_dev_init does vhost_poll_init for 
> example.
>
> - At open time, we don't know if this vhost_dev is going to be part of 
> a multiple vhost_device device or a single one so we need to create at 
> least 3 of them
> - If it is a multiple device we don't know if its the first device 
> being created for the device or the N'th, so we don't know if the 
> dev's vqs will be used for IO or ctls/evts, so we have to create all 3.
>
> When we get the first VHOST_SCSI_SET_ENDPOINT call for a new style 
> multiple vhost_dev device, we can use that dev's evt/ctl vqs for 
> events/controls requests. When we get the other 
> VHOST_SCSI_SET_ENDPOINT calls for the multiple vhost_dev device then 
> those dev's evt/ctl vqs will be ignored and we will only use their IO 
> vqs. So we end up with a lot of extra vqs.


Right, so in this case we can use this patch to address this issue 
probably. If evt/ctl vq is not used, we won't even create them.


>
>
> One other question/issue I have is that qemu can open the 
> /dev/vhost-scsi device or it allows tools like libvirtd to open the 
> device and pass in the fd to use.


It allows libvirt to open and pass fds to qemu. This is how multie-queue 
virtio-net is done, libvirt is in charge of opening multiple file 
descriptors and pass them to qemu.


> For the latter case, would we continue to have those tools pass in the 
> leading fd, then have qemu do the other num_queues - 1 
> open(/dev/vhost-scsi) calls? Or do these apps that pass in the fd need 
> to know about all of the fds for some management reason?


Usually qemu is running without privilege. So it depends on the 
management to open the device.

Note that I'm not object your proposal, just want to see if it could be 
done via a more easy way. During the development if multiqueue 
virito-net, something similar as you've done was proposed but we end up 
with the multiple vhost-net fd model which keeps kernel code unchanged.

Thanks

  reply index

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-22  0:34 [PATCH 00/17 V3] vhost: fix scsi cmd handling and cgroup support Mike Christie
2020-10-22  0:34 ` [PATCH 01/17] vhost scsi: add lun parser helper Mike Christie
2020-10-26  3:33   ` Jason Wang
2020-10-22  0:34 ` [PATCH 02/17] vhost: remove work arg from vhost_work_flush Mike Christie
2020-10-22  0:51   ` Chaitanya Kulkarni
2020-10-22  0:34 ` [PATCH 03/17] vhost net: use goto error handling in open Mike Christie
2020-10-22  0:45   ` Chaitanya Kulkarni
2020-10-26  3:34   ` Jason Wang
2020-10-22  0:34 ` [PATCH 04/17] vhost: prep vhost_dev_init users to handle failures Mike Christie
2020-10-22  5:22   ` kernel test robot
2020-10-23 16:15   ` Mike Christie
2020-11-02  5:57   ` Jason Wang
2020-11-03 10:04   ` Dan Carpenter
2020-10-22  0:34 ` [PATCH 05/17] vhost: move vq iovec allocation to dev init time Mike Christie
2020-10-22  0:34 ` [PATCH 06/17] vhost: support delayed vq creation Mike Christie
2020-10-22  0:34 ` [PATCH 07/17] vhost scsi: support delayed IO " Mike Christie
2020-10-26  3:51   ` Jason Wang
2020-10-27  5:47     ` Mike Christie
2020-10-28  1:55       ` Jason Wang [this message]
2020-10-30  8:47       ` Michael S. Tsirkin
2020-10-30 16:30         ` Mike Christie
2020-10-30 17:26           ` Mike Christie
2020-11-01 22:06         ` Mike Christie
2020-11-02  6:36         ` Jason Wang
2020-11-02  6:49           ` Jason Wang
2020-11-02 16:19             ` Mike Christie
2020-10-22  0:34 ` [PATCH 08/17] vhost scsi: alloc cmds per vq instead of session Mike Christie
2020-10-22  0:34 ` [PATCH 09/17] vhost scsi: fix cmd completion race Mike Christie
2020-10-27 13:07   ` Maurizio Lombardi
2020-10-30  8:51   ` Michael S. Tsirkin
2020-10-30 16:04     ` Paolo Bonzini
2020-10-22  0:34 ` [PATCH 10/17] vhost scsi: Add support for LUN resets Mike Christie
2020-10-22  0:34 ` [PATCH 11/17] vhost scsi: remove extra flushes Mike Christie
2020-10-22  0:34 ` [PATCH 12/17] vhost poll: fix coding style Mike Christie
2020-10-22  0:39   ` Chaitanya Kulkarni
2020-10-22  0:34 ` [PATCH 13/17] vhost: support multiple worker threads Mike Christie
2020-10-22  0:35 ` [PATCH 14/17] vhost: poll support support multiple workers Mike Christie
2020-10-22  0:35 ` [PATCH 15/17] host: support delayed vq creation Mike Christie
2020-10-22  0:50   ` Mike Christie
2020-10-22  0:35 ` [PATCH 16/17] vhost scsi: multiple worker support Mike Christie
2020-10-22  0:35 ` [PATCH 17/17] vhost scsi: drop submission workqueue Mike Christie
2020-10-29 21:47 ` [PATCH 00/17 V3] vhost: fix scsi cmd handling and cgroup support Michael S. Tsirkin
2020-10-29 22:19   ` Mike Christie

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=adcaeb64-b1da-2195-6656-ecc19f12e5ed@redhat.com \
    --to=jasowang@redhat.com \
    --cc=linux-scsi@vger.kernel.org \
    --cc=martin.petersen@oracle.com \
    --cc=michael.christie@oracle.com \
    --cc=mst@redhat.com \
    --cc=pbonzini@redhat.com \
    --cc=stefanha@redhat.com \
    --cc=target-devel@vger.kernel.org \
    --cc=virtualization@lists.linux-foundation.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Target-devel archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/target-devel/0 target-devel/git/0.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 target-devel target-devel/ https://lore.kernel.org/target-devel \
		target-devel@vger.kernel.org
	public-inbox-index target-devel

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.target-devel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git