On Mon, 9 Sep 2019 11:41:45 +0000 "Liu, Yi L" <yi.l.liu@xxxxxxxxx> 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. A concern with 1) is that we specifically made the vfio-mdev-pci driver a sample driver to avoid user confusion over when to use vfio-pci vs when to use vfio-mdev-pci. This use case suggests vfio-mdev-pci becoming a peer of vfio-pci when really I think it was meant only as a demo of IOMMU backed mdev devices and perhaps a starting point for vendors wanting to create an mdev wrapper around real hardware. I had assumed that in the latter case, the sample driver would be forked. Do these new suggestions indicate we're deprecating vfio-pci? I'm not necessarily in favor of that. Couldn't we also have device specific extensions of vfio-pci that could provide migration support for a physical device? Do we really want to add the usage burden of the mdev sysfs interface if we're only adding migration to a VF? Maybe instead we should add common helpers for migration that could be used by either vfio-pci or vendor specific mdev drivers. Ideally I think that if we're not trying to multiplex a device into multiple mdevs or trying to supplement a device that would be incomplete without mdev, and only want to enable migration for a PF/VF, we'd bind it to vfio-pci and those features would simply appear for device we've enlightened vfio-pci to migrate. Thanks, Alex
