Re: [PATCH 07/17] vhost scsi: support delayed IO vq creation

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Oct 27, 2020 at 12:47:34AM -0500, 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.

The issue Jason's hinting at is how can admins control the amount
of resources a given qemu instance can consume?
After all vhost vqs all live in host kernel memory ...
Limiting # of open fds would be one way to do that ...

The need to share event/control vqs between devices is a problem though,
and sending lots of ioctls on things like reset is also not that elegant.
Jason, did you have a good solution in mind?

> 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. 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?

They know about all the fds, for resource control and priveledge
separation reasons.

-- 
MST