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

[Longpeng (Mike, Cloud Infrastructure Service Product Dept.)]

> -----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.

> Stefan