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

Yan Zhao
2019-12-05T08:59:27Z

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
> >
>