On 11/18/20 1:54 AM, Jason Wang wrote:
On 2020/11/18 下午2:57, Mike Christie wrote:
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?
I think it means when a vq is disabled, vhost won't process any request
from that virtqueue.
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
mlx5_vdpa_set_vq_ready
where it can do some more work in the disable case?
For vDPA, it would be more complicated.
E.g for IFCVF, it just delay the setting of queue_enable when it get
DRIVER_OK. Technically it can passthrough the queue_enable to the
hardware as what mlx5e did.
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 ioctl.
Yes, vhost support backend capability. When userspace negotiate the new
capability, we should depend on SET_VRING_ENABLE, if not we can do
vhost_vq_is_enable().
And then in vhost_net_set_backend do we call vhost_vq_is_enabled()?
What is done
for disable then?
It needs more thought, but the question is not specific to
SET_VRING_ENABLE. Consider guest may zero ring address as well.
For disabling, we can simply flush the work and disable all the polls.
It doesn't seem to buy a lot of new functionality. Is it just
so we follow the spec?
My understanding is that, since spec defines queue_enable, we should
support it in vhost. And we can piggyback the delayed vq creation with
this feature. Otherwise we will duplicate the function if we want to
support queue_enable.
I had actually given up on the delayed vq creation goal. I'm still not
sure how it's related to ENABLE and I think it gets pretty gross.
1. If we started from a semi-clean slate, and used the ENABLE ioctl more
like a CREATE ioctl, and did the ENABLE after vhost dev open() but
before any other ioctls, we can allocate the vq when we get the ENABLE
ioctl. This fixes the issue where vhost scsi is allocating 128 vqs at
open() time. We can then allocate metadata like the iovecs at ENABLE
time or when we get a setup ioctl that is related to the metadata, so it
fixes that too.
That makes sense how ENABLE is related to delayed vq allocation and why
we would want it.
If we now need to support old tools though, then you lose me. To try and
keep the code paths using the same code, then at vhost dev open() time
do we start vhost_dev_init with zero vqs like with the allocate at
ENABLE time case? Then when we get the first vring or dev ioctl, do we
allocate the vq and related metadata? If so, the ENABLE does not buy us
a lot since we get the delayed allocation from the compat code. Also
this compat case gets really messy when we are delaying the actual vq
and not just the metadata.
If for the compat case, we keep the code that before/during
vhost_dev_init allocates all the vqs and does the initialization, then
we end up with 2 very very different code paths. And we also need a new
modparam or something to tell the drivers to do the old or new open()
behavior.
2. If we do an approach that is less invasive to the kernel for the
compat case, and do the ENABLE ioctl after other vring ioctl calls then
that would not work for the delayed vq allocation goal since the ENABLE
call is too late.
Or do you want it work more like mlx5_vdpa_set_vq_ready? For this in
vhost_ring_ioctl
when we get the new ioctl we would call into the drivers and have it
start queues
and stop queues? For enable, what we you do for net for this case?
Net is something different, we can simply use SET_BACKEND to disable a
specific virtqueue without introducing new ioctls. Notice that, net mq
is kind of different with scsi which have a per queue pair vhost device,
and the API allows us to set backend for a specific virtqueue.
That's one of the things I am trying to understand. It sounds like
ENABLE is not useful to net. Will net even use/implement the ENABLE
ioctl or just use the SET_BACKEND? What about vsock?
For net it sounds like it's just going to add an extra code path if you
support it.
For disable,
would you do something like vhost_net_stop_vq (we don't free up
anything allocated
in vhost_vring_ioctl calls, but we can stop what we setup in the net
driver)?
It's up to you, if you think you should free the resources you can do that.
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?
I think SET_VRING_ENABLE is more useful for SCSI since it have a model
of multiple vqs per vhost device.
That is why I was asking about if you were going to change net.
It would have been useful for scsi if we had it when mq support was
added and we don't have to support old tools. But now, if enable=true,
is only going to be something where we set some bit so later when
VHOST_SCSI_SET_ENDPOINT is run it we can do what we are already doing
its just extra code. This patch:
https://www.spinics.net/lists/linux-scsi/msg150151.html
would work without the ENABLE ioctl I mean.
And if you guys want to do the completely new interface, then none of
this matters I guess :)
For disable see below.
My issue/convern is that in general these calls seems useful, but we
don't really
need them for scsi because vhost scsi is already stuck creating vqs
like how it does
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 extra
check and extra code.
And for the mlx5_vdpa_set_vq_ready type of design, it doesn't seem
like it's going
to happen a lot where the admin is going to want to remove vqs from a
running device.
In this case, qemu may just disable the queues of vhost-scsi via
SET_VRING_ENABLE and then we can free resources?
Some SCSI background in case it doesn't work like net:
-------
When the user sets up mq for vhost-scsi/virtio-scsi, for max perf and no
cares about mem use they would normally set num_queues based on the
number of vCPUs and MSI-x vectors. I think the default in qemu now is to
try and detect that value.
When the virtio_scsi driver is loaded into the guest kernel, it takes
the num_queues value and tells the scsi/block mq layer to create
num_queues multiqueue hw queues.
------
I was trying to say in the previous email that is if all we do is set
some bits to indicate the queue is disabled, free its resources, stop
polling/queueing in the scsi/target layer, flush etc, it does not seem
useful. I was trying to ask when would a user only want this behavior?
I think we need an extra piece where the guests needs to be modified to
handle the queue removal or the block/scsi layers would still send IO
and we would get IO errors. Without this it seems like some extra code
that we will not use.
And then if we are going to make disable useful like this, what about
enable? We would want to the reverse where we add the queue and the
guest remaps the mq to hw queue layout. To do this, enable has to do
more than just set some bits. There is also an issue with how it would
need to interact with the SET_BACKEND
(VHOST_SCSI_SET_ENDPOINT/VHOST_SCSI_CLEAR_ENDPOINT for scsi) calls.
I think if we wanted the ENABLE ioctl to work like this then that is not
related to my patches and I like I've written before I think my patches
do not need the ENABLE ioctl in general. We could add the patch where we
create the workers threads from VHOST_SCSI_SET_ENDPOINT. And if we ever
add this queue hotplug type of code, then the worker thread would just
get moved/rearranged with the other vq modification code in
vhost_scsi_set_endpoint/vhost_scsi_clear_endpoint.
We could also go the new threading interface route, and also do the
ENABLE ioctl separately.
