Re: [RFC] vhost-vdpa-net: add vhost-vdpa-net host device support

[Jason Wang]

On Thu, Dec 16, 2021 at 5:10 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Thu, Dec 16, 2021 at 11:01:40AM +0800, Jason Wang wrote:
> > On Wed, Dec 15, 2021 at 6:07 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > >
> > > On Wed, Dec 15, 2021 at 11:18:05AM +0800, Jason Wang wrote:
> > > > On Tue, Dec 14, 2021 at 9:11 PM Stefan Hajnoczi <stefanha@redhat.com> 
> > > > wrote:
> > > > >
> > > > > On Tue, Dec 14, 2021 at 10:22:53AM +0800, Jason Wang wrote:
> > > > > > On Mon, Dec 13, 2021 at 11:14 PM Stefan Hajnoczi 
> > > > > > <stefanha@redhat.com> wrote:
> > > > > > >
> > > > > > > On Mon, Dec 13, 2021 at 10:47:00AM +0800, Jason Wang wrote:
> > > > > > > > On Sun, Dec 12, 2021 at 5:30 PM Michael S. Tsirkin 
> > > > > > > > <mst@redhat.com> wrote:
> > > > > > > > >
> > > > > > > > > On Sat, Dec 11, 2021 at 03:00:27AM +0000, Longpeng (Mike, 
> > > > > > > > > Cloud Infrastructure Service Product Dept.) wrote:
> > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > > -----Original Message-----
> > > > > > > > > > > From: Stefan Hajnoczi [mailto:stefanha@redhat.com]
> > > > > > > > > > > Sent: Thursday, December 9, 2021 5:17 PM
> > > > > > > > > > > To: Longpeng (Mike, Cloud Infrastructure Service Product 
> > > > > > > > > > > Dept.)
> > > > > > > > > > > <longpeng2@huawei.com>
> > > > > > > > > > > Cc: jasowang@redhat.com; mst@redhat.com; parav@nvidia.com;
> > > > > > > > > > > xieyongji@bytedance.com; sgarzare@redhat.com; Yechuan 
> > > > > > > > > > > <yechuan@huawei.com>;
> > > > > > > > > > > Gonglei (Arei) <arei.gonglei@huawei.com>; 
> > > > > > > > > > > qemu-devel@nongnu.org
> > > > > > > > > > > Subject: Re: [RFC] vhost-vdpa-net: add vhost-vdpa-net 
> > > > > > > > > > > host device support
> > > > > > > > > > >
> > > > > > > > > > > On Wed, Dec 08, 2021 at 01:20:10PM +0800, Longpeng(Mike) 
> > > > > > > > > > > wrote:
> > > > > > > > > > > > From: Longpeng <longpeng2@huawei.com>
> > > > > > > > > > > >
> > > > > > > > > > > > Hi guys,
> > > > > > > > > > > >
> > > > > > > > > > > > This patch introduces vhost-vdpa-net device, which is 
> > > > > > > > > > > > inspired
> > > > > > > > > > > > by vhost-user-blk and the proposal of vhost-vdpa-blk 
> > > > > > > > > > > > device [1].
> > > > > > > > > > > >
> > > > > > > > > > > > I've tested this patch on Huawei's offload card:
> > > > > > > > > > > > ./x86_64-softmmu/qemu-system-x86_64 \
> > > > > > > > > > > >     -device 
> > > > > > > > > > > > vhost-vdpa-net-pci,vdpa-dev=/dev/vhost-vdpa-0
> > > > > > > > > > > >
> > > > > > > > > > > > For virtio hardware offloading, the most important 
> > > > > > > > > > > > requirement for us
> > > > > > > > > > > > is to support live migration between offloading cards 
> > > > > > > > > > > > from different
> > > > > > > > > > > > vendors, the combination of netdev and virtio-net seems 
> > > > > > > > > > > > too heavy, we
> > > > > > > > > > > > prefer a lightweight way.
> > > > > > > > > > > >
> > > > > > > > > > > > Maybe we could support both in the future ? Such as:
> > > > > > > > > > > >
> > > > > > > > > > > > * Lightweight
> > > > > > > > > > > >  Net: vhost-vdpa-net
> > > > > > > > > > > >  Storage: vhost-vdpa-blk
> > > > > > > > > > > >
> > > > > > > > > > > > * Heavy but more powerful
> > > > > > > > > > > >  Net: netdev + virtio-net + vhost-vdpa
> > > > > > > > > > > >  Storage: bdrv + virtio-blk + vhost-vdpa
> > > > > > > > > > > >
> > > > > > > > > > > > [1] 
> > > > > > > > > > > > https://www.mail-archive.com/qemu-devel@nongnu.org/msg797569.html
> > > > > > > > > > >
> > > > > > > > > > > Stefano presented a plan for vdpa-blk at KVM Forum 2021:
> > > > > > > > > > > https://kvmforum2021.sched.com/event/ke3a/vdpa-blk-unified-hardware-and-sof
> > > > > > > > > > > tware-offload-for-virtio-blk-stefano-garzarella-red-hat
> > > > > > > > > > >
> > > > > > > > > > > It's closer to today's virtio-net + vhost-net approach 
> > > > > > > > > > > than the
> > > > > > > > > > > vhost-vdpa-blk device you have mentioned. The idea is to 
> > > > > > > > > > > treat vDPA as
> > > > > > > > > > > an offload feature rather than a completely separate code 
> > > > > > > > > > > path that
> > > > > > > > > > > needs to be maintained and tested. That way QEMU's block 
> > > > > > > > > > > layer features
> > > > > > > > > > > and live migration work with vDPA devices and re-use the 
> > > > > > > > > > > virtio-blk
> > > > > > > > > > > code. The key functionality that has not been implemented 
> > > > > > > > > > > yet is a "fast
> > > > > > > > > > > path" mechanism that allows the QEMU virtio-blk device's 
> > > > > > > > > > > virtqueue to be
> > > > > > > > > > > offloaded to vDPA.
> > > > > > > > > > >
> > > > > > > > > > > The unified vdpa-blk architecture should deliver the same 
> > > > > > > > > > > performance
> > > > > > > > > > > as the vhost-vdpa-blk device you mentioned but with more 
> > > > > > > > > > > features, so I
> > > > > > > > > > > wonder what aspects of the vhost-vdpa-blk idea are 
> > > > > > > > > > > important to you?
> > > > > > > > > > >
> > > > > > > > > > > QEMU already has vhost-user-blk, which takes a similar 
> > > > > > > > > > > approach as the
> > > > > > > > > > > vhost-vdpa-blk device you are proposing. I'm not against 
> > > > > > > > > > > the
> > > > > > > > > > > vhost-vdpa-blk approach in priciple, but would like to 
> > > > > > > > > > > understand your
> > > > > > > > > > > requirements and see if there is a way to collaborate on 
> > > > > > > > > > > one vdpa-blk
> > > > > > > > > > > implementation instead of dividing our efforts between 
> > > > > > > > > > > two.
> > > > > > > > > > >
> > > > > > > > > >
> > > > > > > > > > We prefer a simple way in the virtio hardware offloading 
> > > > > > > > > > case, it could reduce
> > > > > > > > > > our maintenance workload, we no need to maintain the 
> > > > > > > > > > virtio-net, netdev,
> > > > > > > > > > virtio-blk, bdrv and ... any more. If we need to support 
> > > > > > > > > > other vdpa devices
> > > > > > > > > > (such as virtio-crypto, virtio-fs) in the future, then we 
> > > > > > > > > > also need to maintain
> > > > > > > > > > the corresponding device emulation code?
> > > > > > > > > >
> > > > > > > > > > For the virtio hardware offloading case, we usually use the 
> > > > > > > > > > vfio-pci framework,
> > > > > > > > > > it saves a lot of our maintenance work in QEMU, we don't 
> > > > > > > > > > need to touch the device
> > > > > > > > > > types. Inspired by Jason, what we really prefer is 
> > > > > > > > > > "vhost-vdpa-pci/mmio", use it to
> > > > > > > > > > instead of the vfio-pci, it could provide the same 
> > > > > > > > > > performance as vfio-pci, but it's
> > > > > > > > > > *possible* to support live migrate between offloading cards 
> > > > > > > > > > from different vendors.
> > > > > > > > >
> > > > > > > > > OK, so the features you are dropping would be migration 
> > > > > > > > > between
> > > > > > > > > a vdpa, vhost and virtio backends. I think given 
> > > > > > > > > vhost-vdpa-blk is seems
> > > > > > > > > fair enough... What do others think?
> > > > > > > >
> > > > > > > > I think it should be fine, and it would be even better to make 
> > > > > > > > it not
> > > > > > > > specific to device type.
> > > > > > >
> > > > > > > That's an interesting idea. A generic vDPA VirtIODevice could 
> > > > > > > exposed as
> > > > > > >
> > > > > > >   --device vhost-vdpa-pci,
> > > > > > >            [vhostfd=FD,|
> > > > > > >             vhostpath=/dev/vhost-vdpa-N]
> > > > > > >
> > > > > > > (and for virtio-mmio and virtio-ccw too).
> > > > > > >
> > > > > > > I don't think this is possible yet because the vhost_vdpa ioctls 
> > > > > > > are
> > > > > > > missing some introspection functionality. Here is what I found:
> > > > > > > - Device ID: ok, use VHOST_VDPA_GET_DEVICE_ID
> > > > > > > - Device feature bits: ok, use VHOST_GET_BACKEND_FEATURES
> > > > > > > - Configuration space size: missing, need ioctl for 
> > > > > > > ops->get_config_size()
> > > > > >
> > > > > > Any specific reason that we need this considering we've already had
> > > > > > VHOST_VDPA_GET_CONFIG and we do the size validation there?
> > > > >
> > > > > QEMU's virtio_init() takes a size_t config_size argument. We need to
> > > > > determine the size of the vhost_vdpa's configuration space in order to
> > > > > create the VirtIODevice in QEMU.
> > > > >
> > > > > Do you mean probing by checking for the VHOST_VDPA_GET_CONFIG -E2BIG
> > > > > return value? It's hacky but I guess it's possible to do a binary 
> > > > > search
> > > > > that calls VHOST_VDPA_GET_CONFIG each iteration and reduces the size 
> > > > > if
> > > > > -E2BIG is returned or increases the size otherwise.
> > > > >
> > > > > Or do you mean re-writing QEMU's hw/virtio/virtio.c to allow the
> > > > > VirtIODevice to override the size and we pass accesses through to
> > > > > vhost_vdpa. That way it might be possible to avoid fetching the
> > > > > configuration space size at startup, but I'm not sure this will work
> > > > > because QEMU might depend on knowing the exact size (e.g. live
> > > > > migration).
> > > >
> > > > Good point, so looking at virtio-blk it has:
> > > >
> > > >     virtio_blk_set_config_size(s, s->host_features);
> > > >     virtio_init(vdev, "virtio-blk", VIRTIO_ID_BLOCK, s->config_size);
> > > >
> > > > I think here virtio-blk/net should check the vhost-vdpa features here
> > > > and fail if they are not the same?
> > >
> > > The vhost feature bit code in QEMU is complicated and I can't respond
> > > without investing too much time studying it :).
> > >
> > > > This looks better than overriding the config_size with what vhost-vdpa
> > > > provides since it can override the features that the cli tries to
> > > > enable.
> > >
> > > I'm thinking about the generic --device vhost-vdpa idea. QEMU should not
> > > require knowledge of the device feature bits in that case, so it cannot
> > > calculate the configuration space size.
> >
> > In this case, it looks to me the config size could be deduced from
> > VHOST_VDPA_GET_FEATURES?
>
> I think we're talking about different things, see below...
>
> > >
> > > > >
> > > > > > > - Max virtqueue size: ok, VHOST_VDPA_GET_VRING_NUM
> > > > > > > - Number of virtqueues: probe using VHOST_GET_VRING_BASE?
> > > > > >
> > > > > > I'm not sure whether or not we need this and it seems not necessary
> > > > > > since it can be deduced from the config space and features.
> > > > >
> > > > > It can only be deduced in a device-specific way (net, blk, etc). I 
> > > > > can't
> > > > > think of a way to detect the number of virtqueues for an arbitrary
> > > > > VIRTIO device from the features bits and configuration space contents.
> > > >
> > > > Yes, I'm not against this idea but it looks to me it works even without 
> > > > this.
> > > >
> > > > Modern PCI has num_queues but we don't have things like this in MMIO
> > > > and legacy PCI.
> > >
> > > Even if the VIRTIO hardware interface doesn't expose this information to
> > > the guest, QEMU's VirtIODevice API needs it. Device emulation code must
> > > call virtio_add_queue() to expose virtqueues to the guest.
> >
> > We don't need this for current multiqueue virtio-net with vhost-vdpa
> > since the queue num were deduced from the VHOST_VDPA_GET_CONFIG during
> > the initialization of vhost-vdpa backend.
> >
> > If we are talking about generic vhost-vdpa-pci, we don't need
> > virtio_add_queue() in this case.
>
> When I say --device vhost-vdpa I mean a VirtIODevice in QEMU that takes
> any /dev/vhost-vdpa-N and exposes the device to the guest (over
> virtio-pci, virtio-mmio, or virtio-ccw). It's generic because it has no
> knowledge of specific device types. This means new device types can be
> added without modifying QEMU.
>
> I think the model you are describing is not generic because it relies on
> knowledge of specific device types (net, blk, scsi, etc) so it can
> interpret feature bits and configuration space fields.

Yes, but what I meant is that in this case qemu can simply relay the
set/get config to vhost-vdpa. And the guest driver can enumerate the
number of queues correctly depending on his own knowledge.

>
> When you originally said "it would be even better to make it not
> specific to device type" I thought you meant a generic --device
> vhost-vdpa and that's what I've been describing, but in your recent
> replies I guess you have a different model in mind.
>
> Are there reasons why the generic model won't work?

I think not.

One thing comes to my mind is that since we provide num_queues via
modern virtio-pci, this is probably another call for having the API
you described.

For the general vhost-vdpa backend, the only thing that may block us
is the migration. If we want to make vhost-vdpa type independent, we
need first investigate the independent migration facility in virtio
spec which is still suspicious.

Thanks

>
> Stefan