Re: mdev live migration support with vfio-mdev-pci

Jason Wang
2019-09-16T08:55:31Z

On 2019/9/9 下午7:41, Liu, Yi L wrote:
> Hi Alex,
>
> Recently, we had an internal discussion on mdev live migration support
> for SR-IOV. The usage is to wrap VF as mdev and make it migrate-able
> when passthru to VMs. It is very alike with the vfio-mdev-pci sample
> driver work which also wraps PF/VF as mdev. But there is gap. Current
> vfio-mdev-pci driver is a generic driver which has no ability to support
> customized regions. e.g. state save/restore or dirty page region which is
> important in live migration. To support the usage, there are two directions:
>
> 1) extend vfio-mdev-pci driver to expose interface, let vendor specific
> in-kernel module (not driver) to register some ops for live migration.
> Thus to support customized regions. In this direction, vfio-mdev-pci
> driver will be in charge of the hardware. The in-kernel vendor specific
> module is just to provide customized region emulation.
> - Pros: it will be helpful if we want to expose some user-space ABI in
> future since it is a generic driver.
> - Cons: no apparent cons per me, may keep me honest, my folks.
>
> 2) further abstract out the generic parts in vfio-mdev-driver to be a library
> and let vendor driver to call the interfaces exposed by this library. e.g.
> provides APIs to wrap a VF as mdev and make a non-singleton iommu
> group to be vfio viable when a vendor driver wants to wrap a VF as a
> mdev. In this direction, device driver still in charge of hardware.
> - Pros: devices driver still owns the device, which looks to be more
> "reasonable".
> - Cons: no apparent cons, may be unable to have unified user space ABI if
> it's needed in future.
>
> Any thoughts on the above usage and the two directions? Also, Kevin, Yan,
> Shaopeng could keep me honest if anything missed.
>
> Best Wishes,
> Yi Liu

Actually, we had option 3:

3) High level abstraction of the device instead of a bus specific one
(e.g PCI). For hardware that can do virtio on its datapath, we want to
go this way. This means, we won't expose a pci device for userspace,
instead, we will expose a vhost device for userspace which already had
API for e.g dirty page logging and vring state set/get etc.

Thanks