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

[Stefan Hajnoczi <stefanha@redhat.com>]

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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()
- Max virtqueue size: ok, VHOST_VDPA_GET_VRING_NUM
- Number of virtqueues: probe using VHOST_GET_VRING_BASE?

I think it's worth adding the missing introspection so that VMMs like
QEMU can implement a generic vDPA device.

Stefan

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