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

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 9 Sep 2019 11:41:45 +0000
"Liu, Yi L" <yi.l.liu@intel.com> 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