On Fri, 13 Sep 2019 00:28:25 +0000 "Tian, Kevin" <kevin.tian@xxxxxxxxx> wrote: > > From: Alex Williamson > > Sent: Thursday, September 12, 2019 10:41 PM > > > > 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, > > > > That would be better and simpler. We thought you may want to keep > current vfio-pci intact. :-) btw do you prefer to putting device specific > migration logic within VFIO, or building some mechanism for PF/VF driver > to register and handle? The former is fully constrained with VFIO but > moving forward may get complex. The latter keeps the VFIO clean and > reuses existing driver logic thus simpler, just that PF/VF driver enters > a special mode in which it's not bound to the PF/VF device (vfio-pci is > the actual driver) and simply acts as callbacks to handler device specific > migration request. I'm not sure how this native device driver registration would work, is seems troublesome for the user. vfio-pci already has optional support for IGD extensions. I imagine it would be similar to that, but we may try to modularize it within vfio-pci, perhaps similar to how Eric supports resets on vfio-platform. There's also an issue with using the native driver that users cannot then blacklist the native driver if they only intend to use the device with vfio-pci. It's probably best to keep it contained, but modular within vfio-pci. Thanks, Alex
