On Thu, Apr 08, 2021 at 06:12:49PM +0800, Xie Yongji wrote: > Since we already have some ways to emulate vDPA block device > in kernel[1] or userspace[2]. This series tries to introduce a > new vhost-vdpa block device for that. To use it, we can add > something like: > > qemu-system-x86_64 \ > -device vhost-vdpa-blk-pci,vdpa-dev=/dev/vhost-vdpa-0
This device is similar to vhost-user-blk. QEMU does not see it as a block device so storage migration, I/O throttling, image formats, etc are not supported. Stefano Garzarella and I discussed how vdpa-blk devices could integrate more deeply with QEMU's block layer. The QEMU block layer could be enabled only when necessary and otherwise bypassed for maximum performance. This alternative approach is similar to how vhost-net is implemented in QEMU. A BlockDriver would handle the vdpa-blk device and the regular virtio-blk-pci device would still be present. The virtqueues could be delegated to the vdpa-blk device in order to bypass the QEMU block layer. I wanted to mention this since it's likely that this kind of vdpa-blk device implementation will be posted in the future and you might be interested. It makes live migration with non-shared storage possible, for example. An issue with vhost-user-blk is that the ownership of qdev properties and VIRTIO Configuration Space fields was not clearly defined. Some things are handled by QEMU's vhost-user-blk code, some things are handled by the vhost-user device backend, and some things are negotiated between both entities. This patch series follows the existing vhost-user-blk approach, which I think will show serious issues as the device is more widely used and whenever virtio-blk or the implementation is extended with new features. It is very hard to provide backwards compatibility with the current approach where the ownership of qdev properties and VIRTIO Configuration Space fields is ad-hoc and largely undefined. Since vDPA has VIRTIO Configuration Space APIs, I suggest that the vhost-vDPA device controls the entire configuration space. QEMU should simply forward accesses between the guest and vhost-vdpa. Regarding qdev properties, it's a little trickier because QEMU needs to do the emulated VIRTIO device setup (allocating virtqueues, setting their sizes, etc). Can QEMU query this information from the vDPA device? If not, which qdev properties are read-only and must match the configuration of the vDPA device and which are read-write and can control the vDPA device? Stefan
signature.asc
Description: PGP signature
