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

From: Jason Wang
Subject: Re: [RFC PATCH 0/9] Introduce mediate ops in vfio-pci
Date: Wed, 18 Dec 2019 10:36:11 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 2019/12/12 下午1:47, Yan Zhao wrote:
On Thu, Dec 12, 2019 at 11:48:25AM +0800, Jason Wang wrote:
On 2019/12/6 下午8:49, Yan Zhao wrote:
On Fri, Dec 06, 2019 at 05:40:02PM +0800, Jason Wang wrote:
On 2019/12/6 下午4:22, Yan Zhao wrote:
On Thu, Dec 05, 2019 at 09:05:54PM +0800, Jason Wang wrote:
On 2019/12/5 下午4:51, Yan Zhao wrote:
On Thu, Dec 05, 2019 at 02:33:19PM +0800, Jason Wang wrote:

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.
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.

hi Jason

not familiar with the way you mentioned. could you elaborate more?

It looks to me that you want to intercept the bar that contains the
head. Then you can figure out the buffers submitted from driver and you
still need to decide a proper time to mark them as dirty.

Not need to be accurate, right? just a superset of real dirty bitmap is

If the superset is too large compared with the dirty pages, it will lead a lot of side effects.

What I meant is, intercept the interrupt, then you can figure still
figure out the buffers which has been modified by the device and make
them as dirty.

Then there's no need to trap BAR and do decoding/emulation etc.

But it will still be tricky to be correct...

intercept the interrupt is a little hard if post interrupt is enabled..

We don't care about the performance too much in this case. Can we simply disable it?

I think what you worried about here is the timing to mark dirty pages,
right? upon interrupt receiving, you regard DMAs are finished and safe
to make them dirty.
But with BAR trap way, we at least can keep those dirtied pages as dirty
until device stop. Of course we have other methods to optimize it.

I'm not sure this will not confuse the migration converge time estimation.

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->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

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

or previous discussion at
which has kernel side implemetation 

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.

here's the history of vfio live migration:

If you have any concern of this region way, feel free to comment to the
latest v9 patchset:

The patchset here will always keep compatible to there.


   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.

that's why I need to define VFIO_REGION_TYPE_MIGRATION here in this
patchset, as it's not upstreamed yet.
maybe I should make it into a prerequisite patch, indicating it is not
introduced by this patchset


    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).

it's access by host qemu, the same as how linux app access an mmaped
memory. the mmap here is to reduce memory copy from kernel to user.
No need to get mapped into guest.

But copy_to_user() is not a bad choice. If I read the code correctly
only the dirty bitmap was mmaped. This means you probably need to deal
with dcache carefully on some archs. [1]

Note KVM doesn't use shared dirty bitmap, it uses copy_to_user().

[1] https://lkml.org/lkml/2019/4/9/5

on those platforms, mmap can be safely disabled by vendor driver at will.

Then you driver need to detect and behave differently according to the arch.

Also, when mmap is disabled, copy_to_user() is also used in region way.
Any way, please raise you concern in kirti's thread for this common part.

Well, if I read the code correctly, they are all invented in this series. Kirti's thread just a user.

    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
(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
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
vfio-pci is only for PCI of course.

I meant if what propose here makes sense, other bus driver like
vfio-platform may want something similar.

sure they can follow.
2) Function duplicated with mdev and mdev can do even more

could you elaborate how mdev can do solve the above saying problem ?

Well, I think both of us agree the mdev can do what mediate ops did,
mdev device implementation just need to add the direct PCI access 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
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
what is common mdev for vfio-pci? a common mdev parent driver that have
the same implementation as vfio-pci?

The common part is not PCI of course. The common part is the both mdev
and mediate ops want to do some kind of mediation. Mdev is bus agnostic,
but what you propose here is PCI specific but should be bus agnostic as
well. Assume we implement a bug agnostic mediate ops, mdev could be even
built on top.

I believe Alex has already replied the above better than me.
There's actually already a solution of creating only one mdev on top
of each passthrough device, and make mdev share the same iommu group
with it. We've also made an implementation on it already. here's a
sample one made by Yi at https://patchwork.kernel.org/cover/11134695/.

But, as I said, it's desired to re-use vfio-pci directly for SRIOV,
which is straghtforward :)

Can we have a device that is capable of both SRIOV and function slicing?
If yes, does it mean you need to provides two drivers? One for mdev,
another for mediate ops?

what do you mean by "function slicing"? SIOV?

SIOV could be one of the solution.

For vendor driver, in SRIOV
- with mdev approach, two drivers required: one for mdev parent driver on
VF, one for PF driver.
- with mediate ops + vfio-pci: one driver on PF.

in SIOV, only one driver on PF in both case.

The point is, e.g if you have a card that support both SRIOV and SIOV. (I don't think you will ship a card with SIOV only). Then, when SRIOV is used, you need go for mediate ops, when SIOV is used, you need go for e.g mdev. It means you need prepare two set of drivers for migration.

- The most straightforward way to support dirty page tracking is done by
IOMMU instead of device specific operations.

No such IOMMU yet. And all kinds of platforms should be cared, right?

Or the device can track dirty pages by itself, otherwise it would be
very hard to implement dirty page tracking correctly without the help of
switching to software datapath (or maybe you can post the part of BAR0
I think you mixed "correct" and "accurate".

Nope, it depends on how to define correct. We need try our best to be "accurate" instead of  just a superset of dirty pages.

DMA pre-inspection is a long existing term and we have implemented and
verified it in NIC for both precopy and postcopy case.

Interesting, for postcopy, you probably need mediate the whole datapath I guess, but again there's no codes to demonstrate how it works in this series.

Though I can't promise
there's 100% no bug, the method is right.

I fully believe mediation is correct.

Also, whether to trap BARs for dirty page is vendor specific and is not
what should be cared about from this interface part.

The problem is that you introduce a generic interface, it needs to be proved to be useful for other devices. (E.g for virtio migration, it doesn't help).

mediation and dirty page tracking which is missed in this series?)

Currently, that part of code is owned by shaopeng's team. The code I
posted is only for demonstrating how to use the interface. Shaopeng's
team is responsible for upsteam of their part at their timing.

It would be very hard to evaluate a framework without any real users. If possible, please post with driver codes in next version.



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
       patch 7 registers/unregisters VF mediate ops when PF driver
       probes/removes. It specifies its supporting VFs via

       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
       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

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