> -----Original Message----- > From: Zhao, Yan Y > Sent: Monday, September 16, 2019 10:24 AM > To: Alex Williamson <alex.williamson@xxxxxxxxxx> > Cc: Tian, Kevin <kevin.tian@xxxxxxxxx>; Liu, Yi L <yi.l.liu@xxxxxxxxx>; He, > Shaopeng <shaopeng.he@xxxxxxxxx>; Xia, Chenbo <chenbo.xia@xxxxxxxxx>; > kvm@xxxxxxxxxxxxxxx > Subject: Re: mdev live migration support with vfio-mdev-pci > > On Fri, Sep 13, 2019 at 09:54:29AM -0600, Alex Williamson wrote: > > 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 > hi Alex > how about native driver registering a handler which would be chained on every > read/write/ioctl... in vfio_pci_ops? > It will overwrite vfio-pci's default operations if a success is returned in native > driver's handler, or continue to call vfio-pci's default operations if a positive > value (indicating it will not process this param to this op) is returned. > This native driver can be put to passthrough mode in this case, so no need to put > it to blacklist. > > If no handler is registered from native driver, then just default vfio-pci driver is > installed and native driver can be blacklisted. > > Do you think this approach is fine? > > Thanks > Yan For complex device logic (e.g. config/status transfer and dirty page tracking in NIC live migration), this register/ops way will probably have cleaner code base in vfio-pci module. Another benefit is we may have unified user experience for SIOV and SR-IOV. PF acts as the parent device, virtual device in the same native driver acts as mdev interface to hardware SIOV ADI or SR-IOV VF. For SR-IOV case, mdev additionally needs to register and implement the vfio_pci_ops Yan proposed to leverage vfio-pci functionality. Thanks, --Shaopeng
