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

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

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?

> > Stefan