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

[Yan Zhao <yan.y.zhao@intel.com>]

On Thu, Dec 05, 2019 at 02:33:19PM +0800, Jason Wang wrote:
> Hi:
> 
> On 2019/12/5 上午11:24, 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.

> 
> >   (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. The region is only a channel for
QEMU and kernel to communicate information without introducing IOCTLs.


> 
> > (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 :)
> 
> >   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.

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
> > 
>