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Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq

From: Mike Christie
Subject: Re: [PATCH 00/10] vhost/qemu: thread per IO SCSI vq
Date: Wed, 18 Nov 2020 00:57:49 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.3.1

On 11/17/20 11:17 PM, Jason Wang wrote:
> On 2020/11/18 上午12:40, Stefan Hajnoczi wrote:
>> On Thu, Nov 12, 2020 at 05:18:59PM -0600, Mike Christie wrote:
>>> The following kernel patches were made over Michael's vhost branch:
>>> https://urldefense.com/v3/__https://git.kernel.org/pub/scm/linux/kernel/git/mst/vhost.git/log/?h=vhost__;!!GqivPVa7Brio!MzCv3wdRfz5dltunazRWGCeUkMg91pPEOLpIivsebLX9vhYDSi_E1V36e9H0NoRys_hU$
>>> and the vhost-scsi bug fix patchset:
>>> https://urldefense.com/v3/__https://lore.kernel.org/linux-scsi/20201112170008.GB1555653@stefanha-x1.localdomain/T/*t__;Iw!!GqivPVa7Brio!MzCv3wdRfz5dltunazRWGCeUkMg91pPEOLpIivsebLX9vhYDSi_E1V36e9H0NmuPE_m8$
>>> And the qemu patch was made over the qemu master branch.
>>> vhost-scsi currently supports multiple queues with the num_queues
>>> setting, but we end up with a setup where the guest's scsi/block
>>> layer can do a queue per vCPU and the layers below vhost can do
>>> a queue per CPU. vhost-scsi will then do a num_queue virtqueues,
>>> but all IO gets set on and completed on a single vhost-scsi thread.
>>> After 2 - 4 vqs this becomes a bottleneck.
>>> This patchset allows us to create a worker thread per IO vq, so we
>>> can better utilize multiple CPUs with the multiple queues. It
>>> implments Jason's suggestion to create the initial worker like
>>> normal, then create the extra workers for IO vqs with the
>>> VHOST_SET_VRING_ENABLE ioctl command added in this patchset.
>> How does userspace find out the tids and set their CPU affinity?
>> What is the meaning of the new VHOST_SET_VRING_ENABLE ioctl? It doesn't
>> really "enable" or "disable" the vq, requests are processed regardless.
> Actually I think it should do the real "enable/disable" that tries to follow 
> the virtio spec.

What does real mean here? For the vdpa enable call for example, would it be like
ifcvf_vdpa_set_vq_ready where it sets the ready bit or more like 
where it can do some more work in the disable case?

For net and something like ifcvf_vdpa_set_vq_ready's design would we have
vhost_ring_ioctl() set some vhost_virtqueue enable bit. We then have some helper
vhost_vq_is_enabled() and some code to detect if userspace supports the new 
And then in vhost_net_set_backend do we call vhost_vq_is_enabled()? What is done
for disable then? It doesn't seem to buy a lot of new functionality. Is it just
so we follow the spec?

Or do you want it work more like mlx5_vdpa_set_vq_ready? For this in 
when we get the new ioctl we would call into the drivers and have it start 
and stop queues? For enable, what we you do for net for this case? For disable,
would you do something like vhost_net_stop_vq (we don't free up anything 
in vhost_vring_ioctl calls, but we can stop what we setup in the net driver)?
Is this useful for the current net mq design or is this for something like where
you would do one vhost net device with multiple vqs?

My issue/convern is that in general these calls seems useful, but we don't 
need them for scsi because vhost scsi is already stuck creating vqs like how it 
due to existing users. If we do the ifcvf_vdpa_set_vq_ready type of design where
we just set some bit, then the new ioctl does not give us a lot. It's just an 
check and extra code.

And for the mlx5_vdpa_set_vq_ready type of design, it doesn't seem like it's 
to happen a lot where the admin is going to want to remove vqs from a running 
And for both addition/removal for scsi we would need code in virtio scsi to 
hot plug removal/addition of a queue and then redoing the multiqueue mappings 
would be difficult to add with no one requesting it.

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