On Thu, Dec 16, 2021 at 5:10 PM Stefan Hajnoczi <stefa...@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 <stefa...@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 <stefa...@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 > > > > > > <stefa...@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 > > > > > > > > <m...@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:stefa...@redhat.com] > > > > > > > > > > > Sent: Thursday, December 9, 2021 5:17 PM > > > > > > > > > > > To: Longpeng (Mike, Cloud Infrastructure Service Product > > > > > > > > > > > Dept.) > > > > > > > > > > > <longpe...@huawei.com> > > > > > > > > > > > Cc: jasow...@redhat.com; m...@redhat.com; > > > > > > > > > > > pa...@nvidia.com; > > > > > > > > > > > xieyon...@bytedance.com; sgarz...@redhat.com; Yechuan > > > > > > > > > > > <yech...@huawei.com>; > > > > > > > > > > > Gonglei (Arei) <arei.gong...@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 <longpe...@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
