Re: [RFC PATCH 0/9] Introduce mediate ops in vfio-pci

[Jason Wang]

On 2019/12/6 äå4:22, Yan Zhao wrote:
> On Thu, Dec 05, 2019 at 09:05:54PM +0800, Jason Wang wrote:
> > On 2019/12/5 äå4:51, Yan Zhao wrote:
> > > On Thu, Dec 05, 2019 at 02:33:19PM +0800, Jason Wang wrote:
> > > > Hi:
> > > >
> > > > On 2019/12/5 äå11:24, Yan Zhao wrote:
> > > > > For SRIOV devices, VFs are passthroughed into guest directly without host
> > > > > driver mediation. However, when VMs migrating with passthroughed VFs,
> > > > > dynamic host mediation is required to  (1) get device states, (2) get
> > > > > dirty pages. Since device states as well as other critical information
> > > > > required for dirty page tracking for VFs are usually retrieved from PFs,
> > > > > it is handy to provide an extension in PF driver to centralizingly control
> > > > > VFs' migration.
> > > > >
> > > > > Therefore, in order to realize (1) passthrough VFs at normal time, (2)
> > > > > dynamically trap VFs' bars for dirty page tracking and
> > > > A silly question, what's the reason for doing this, is this a must for dirty
> > > > page tracking?
> > > For performance consideration. VFs' bars should be passthoughed at
> > > normal time and only enter into trap state on need.
> >
> > Right, but how does this matter for the case of dirty page tracking?
> Take NIC as an example, to trap its VF dirty pages, software way is
> required to trap every write of ring tail that resides in BAR0.


Interesting, but it looks like we need:
- decode the instruction
- mediate all access to BAR0
All of which seems a great burden for the VF driver. I wonder whether or 
not doing interrupt relay and tracking head is better in this case.


>   There's
> still no IOMMU Dirty bit available.
> > > > >     (3) centralizing
> > > > > VF critical states retrieving and VF controls into one driver, we propose
> > > > > to introduce mediate ops on top of current vfio-pci device driver.
> > > > >
> > > > >
> > > > >                                       _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
> > > > >     __________   register mediate ops|  ___________     ___________    |
> > > > > |          |<-----------------------|     VF    |   |           |
> > > > > | vfio-pci |                      | |  mediate  |   | PF driver |   |
> > > > > |__________|----------------------->|   driver  |   |___________|
> > > > >         |            open(pdev)      |  -----------          |         |
> > > > >         |                                                    |
> > > > >         |                            |_ _ _ _ _ _ _ _ _ _ _ _|_ _ _ _ _|
> > > > >        \|/                                                  \|/
> > > > > -----------                                         ------------
> > > > > |    VF   |                                         |    PF    |
> > > > > -----------                                         ------------
> > > > >
> > > > >
> > > > > VF mediate driver could be a standalone driver that does not bind to
> > > > > any devices (as in demo code in patches 5-6) or it could be a built-in
> > > > > extension of PF driver (as in patches 7-9) .
> > > > >
> > > > > Rather than directly bind to VF, VF mediate driver register a mediate
> > > > > ops into vfio-pci in driver init. vfio-pci maintains a list of such
> > > > > mediate ops.
> > > > > (Note that: VF mediate driver can register mediate ops into vfio-pci
> > > > > before vfio-pci binding to any devices. And VF mediate driver can
> > > > > support mediating multiple devices.)
> > > > >
> > > > > When opening a device (e.g. a VF), vfio-pci goes through the mediate ops
> > > > > list and calls each vfio_pci_mediate_ops->open() with pdev of the opening
> > > > > device as a parameter.
> > > > > VF mediate driver should return success or failure depending on it
> > > > > supports the pdev or not.
> > > > > E.g. VF mediate driver would compare its supported VF devfn with the
> > > > > devfn of the passed-in pdev.
> > > > > Once vfio-pci finds a successful vfio_pci_mediate_ops->open(), it will
> > > > > stop querying other mediate ops and bind the opening device with this
> > > > > mediate ops using the returned mediate handle.
> > > > >
> > > > > Further vfio-pci ops (VFIO_DEVICE_GET_REGION_INFO ioctl, rw, mmap) on the
> > > > > VF will be intercepted into VF mediate driver as
> > > > > vfio_pci_mediate_ops->get_region_info(),
> > > > > vfio_pci_mediate_ops->rw,
> > > > > vfio_pci_mediate_ops->mmap, and get customized.
> > > > > For vfio_pci_mediate_ops->rw and vfio_pci_mediate_ops->mmap, they will
> > > > > further return 'pt' to indicate whether vfio-pci should further
> > > > > passthrough data to hw.
> > > > >
> > > > > when vfio-pci closes the VF, it calls its vfio_pci_mediate_ops->release()
> > > > > with a mediate handle as parameter.
> > > > >
> > > > > The mediate handle returned from vfio_pci_mediate_ops->open() lets VF
> > > > > mediate driver be able to differentiate two opening VFs of the same device
> > > > > id and vendor id.
> > > > >
> > > > > When VF mediate driver exits, it unregisters its mediate ops from
> > > > > vfio-pci.
> > > > >
> > > > >
> > > > > In this patchset, we enable vfio-pci to provide 3 things:
> > > > > (1) calling mediate ops to allow vendor driver customizing default
> > > > > region info/rw/mmap of a region.
> > > > > (2) provide a migration region to support migration
> > > > What's the benefit of introducing a region? It looks to me we don't expect
> > > > the region to be accessed directly from guest. Could we simply extend device
> > > > fd ioctl for doing such things?
> > > You may take a look on mdev live migration discussions in
> > > https://lists.gnu.org/archive/html/qemu-devel/2019-11/msg01763.html
> > >
> > > or previous discussion at
> > > https://lists.gnu.org/archive/html/qemu-devel/2019-02/msg04908.html,
> > > which has kernel side implemetation https://patchwork.freedesktop.org/series/56876/
> > >
> > > generaly speaking, qemu part of live migration is consistent for
> > > vfio-pci + mediate ops way or mdev way.
> >
> > So in mdev, do you still have a mediate driver? Or you expect the parent
> > to implement the region?
> No, currently it's only for vfio-pci.

And specific to PCI.

> mdev parent driver is free to customize its regions and hence does not
> requires this mediate ops hooks.
>
> > > The region is only a channel for
> > > QEMU and kernel to communicate information without introducing IOCTLs.
> >
> > Well, at least you introduce new type of region in uapi. So this does
> > not answer why region is better than ioctl. If the region will only be
> > used by qemu, using ioctl is much more easier and straightforward.
> It's not introduced by me :)
> mdev live migration is actually using this way, I'm just keeping
> compatible to the uapi.


I meant e.g VFIO_REGION_TYPE_MIGRATION.


>  From my own perspective, my answer is that a region is more flexible
> compared to ioctl. vendor driver can freely define the size,

Probably not since it's an ABI I think.

>   mmap cap of
> its data subregion.

It doesn't help much unless it can be mapped into guest (which I don't 
think it was the case here).

>   Also, there're already too many ioctls in vfio.

Probably not :) We had a brunch of subsystems that have much more 
ioctls than VFIO. (e.g DRM)

> > > > > (3) provide a dynamic trap bar info region to allow vendor driver
> > > > > control trap/untrap of device pci bars
> > > > >
> > > > > This vfio-pci + mediate ops way differs from mdev way in that
> > > > > (1) medv way needs to create a 1:1 mdev device on top of one VF, device
> > > > > specific mdev parent driver is bound to VF directly.
> > > > > (2) vfio-pci + mediate ops way does not create mdev devices and VF
> > > > > mediate driver does not bind to VFs. Instead, vfio-pci binds to VFs.
> > > > >
> > > > > The reason why we don't choose the way of writing mdev parent driver is
> > > > > that
> > > > > (1) VFs are almost all the time directly passthroughed. Directly binding
> > > > > to vfio-pci can make most of the code shared/reused.
> > > > Can we split out the common parts from vfio-pci?
> > > That's very attractive. but one cannot implement a vfio-pci except
> > > export everything in it as common part :)
> >
> > Well, I think there should be not hard to do that. E..g you can route it
> > back to like:
> >
> > vfio -> vfio_mdev -> parent -> vfio_pci
> it's desired for us to have mediate driver binding to PF device.
> so once a VF device is created, only PF driver and vfio-pci are
> required. Just the same as what needs to be done for a normal VF passthrough.
> otherwise, a separate parent driver binding to VF is required.
> Also, this parent driver has many drawbacks as I mentions in this
> cover-letter.

Well, as discussed, no need to duplicate the code, bar trick should 
still work. The main issues I saw with this proposal is:

1) PCI specific, other bus may need something similar
2) Function duplicated with mdev and mdev can do even more


> > > > >     If we write a
> > > > > vendor specific mdev parent driver, most of the code (like passthrough
> > > > > style of rw/mmap) still needs to be copied from vfio-pci driver, which is
> > > > > actually a duplicated and tedious work.
> > > > The mediate ops looks quite similar to what vfio-mdev did. And it looks to
> > > > me we need to consider live migration for mdev as well. In that case, do we
> > > > still expect mediate ops through VFIO directly?
> > > >
> > > >
> > > > > (2) For features like dynamically trap/untrap pci bars, if they are in
> > > > > vfio-pci, they can be available to most people without repeated code
> > > > > copying and re-testing.
> > > > > (3) with a 1:1 mdev driver which passthrough VFs most of the time, people
> > > > > have to decide whether to bind VFs to vfio-pci or mdev parent driver before
> > > > > it runs into a real migration need. However, if vfio-pci is bound
> > > > > initially, they have no chance to do live migration when there's a need
> > > > > later.
> > > > We can teach management layer to do this.
> > > No. not possible as vfio-pci by default has no migration region and
> > > dirty page tracking needs vendor's mediation at least for most
> > > passthrough devices now.
> >
> > I'm not quite sure I get here but in this case, just tech them to use
> > the driver that has migration support?
> That's a way, but as more and more passthrough devices have demands and
> caps to do migration, will vfio-pci be used in future any more ?


This should not be a problem:
- If we introduce a common mdev for vfio-pci, we can just bind that 
driver always
- The most straightforward way to support dirty page tracking is done by 
IOMMU instead of device specific operations.

Thanks

> Thanks
> Yan
>
> > Thanks
> >
> >
> > > Thanks
> > > Yn
> > >
> > > > Thanks
> > > >
> > > >
> > > > > In this patchset,
> > > > > - patches 1-4 enable vfio-pci to call mediate ops registered by vendor
> > > > >      driver to mediate/customize region info/rw/mmap.
> > > > >
> > > > > - patches 5-6 provide a standalone sample driver to register a mediate ops
> > > > >      for Intel Graphics Devices. It does not bind to IGDs directly but decides
> > > > >      what devices it supports via its pciidlist. It also demonstrates how to
> > > > >      dynamic trap a device's PCI bars. (by adding more pciids in its
> > > > >      pciidlist, this sample driver actually is not necessarily limited to
> > > > >      support IGDs)
> > > > >
> > > > > - patch 7-9 provide a sample on i40e driver that supports Intel(R)
> > > > >      Ethernet Controller XL710 Family of devices. It supports VF precopy live
> > > > >      migration on Intel's 710 SRIOV. (but we commented out the real
> > > > >      implementation of dirty page tracking and device state retrieving part
> > > > >      to focus on demonstrating framework part. Will send out them in future
> > > > >      versions)
> > > > >      patch 7 registers/unregisters VF mediate ops when PF driver
> > > > >      probes/removes. It specifies its supporting VFs via
> > > > >      vfio_pci_mediate_ops->open(pdev)
> > > > >
> > > > >      patch 8 reports device cap of VFIO_PCI_DEVICE_CAP_MIGRATION and
> > > > >      provides a sample implementation of migration region.
> > > > >      The QEMU part of vfio migration is based on v8
> > > > >      https://lists.gnu.org/archive/html/qemu-devel/2019-08/msg05542.html.
> > > > >      We do not based on recent v9 because we think there are still opens in
> > > > >      dirty page track part in that series.
> > > > >
> > > > >      patch 9 reports device cap of VFIO_PCI_DEVICE_CAP_DYNAMIC_TRAP_BAR and
> > > > >      provides an example on how to trap part of bar0 when migration starts
> > > > >      and passthrough this part of bar0 again when migration fails.
> > > > >
> > > > > Yan Zhao (9):
> > > > >      vfio/pci: introduce mediate ops to intercept vfio-pci ops
> > > > >      vfio/pci: test existence before calling region->ops
> > > > >      vfio/pci: register a default migration region
> > > > >      vfio-pci: register default dynamic-trap-bar-info region
> > > > >      samples/vfio-pci/igd_dt: sample driver to mediate a passthrough IGD
> > > > >      sample/vfio-pci/igd_dt: dynamically trap/untrap subregion of IGD bar0
> > > > >      i40e/vf_migration: register mediate_ops to vfio-pci
> > > > >      i40e/vf_migration: mediate migration region
> > > > >      i40e/vf_migration: support dynamic trap of bar0
> > > > >
> > > > >     drivers/net/ethernet/intel/Kconfig            |   2 +-
> > > > >     drivers/net/ethernet/intel/i40e/Makefile      |   3 +-
> > > > >     drivers/net/ethernet/intel/i40e/i40e.h        |   2 +
> > > > >     drivers/net/ethernet/intel/i40e/i40e_main.c   |   3 +
> > > > >     .../ethernet/intel/i40e/i40e_vf_migration.c   | 626 ++++++++++++++++++
> > > > >     .../ethernet/intel/i40e/i40e_vf_migration.h   |  78 +++
> > > > >     drivers/vfio/pci/vfio_pci.c                   | 189 +++++-
> > > > >     drivers/vfio/pci/vfio_pci_private.h           |   2 +
> > > > >     include/linux/vfio.h                          |  18 +
> > > > >     include/uapi/linux/vfio.h                     | 160 +++++
> > > > >     samples/Kconfig                               |   6 +
> > > > >     samples/Makefile                              |   1 +
> > > > >     samples/vfio-pci/Makefile                     |   2 +
> > > > >     samples/vfio-pci/igd_dt.c                     | 367 ++++++++++
> > > > >     14 files changed, 1455 insertions(+), 4 deletions(-)
> > > > >     create mode 100644 drivers/net/ethernet/intel/i40e/i40e_vf_migration.c
> > > > >     create mode 100644 drivers/net/ethernet/intel/i40e/i40e_vf_migration.h
> > > > >     create mode 100644 samples/vfio-pci/Makefile
> > > > >     create mode 100644 samples/vfio-pci/igd_dt.c