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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: Thu, 12 Dec 2019 12:09:48 +0800
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 2019/12/7 上午1:42, Alex Williamson wrote:
On Fri, 6 Dec 2019 17:40:02 +0800
Jason Wang <address@hidden> 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.
This sounds like a NIC specific solution, I believe the goal here is to
allow any device type to implement a partial mediation solution, in
this case to sufficiently track the device while in the migration
saving state.

I suspect there's a solution that can work for any device type. E.g for virtio, avail index (head) doesn't belongs to any BAR and device may decide to disable doorbell from guest. So did interrupt relay since driver may choose to disable interrupt from device. In this case, the only way to track dirty pages correctly is to switch to software datapath.

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.
What's PCI specific?  The implementation, yes, it's done in the bus
vfio bus driver here but all device access is performed by the bus
driver.  I'm not sure how we could introduce the intercept at the
vfio-core level, but I'm open to suggestions.

I haven't thought this too much, but if we can intercept at core level, it basically can do what mdev can do right now.

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.


  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.
I think Kirti's thread proposing the migration interface is a better
place for this discussion, I believe Yan has already linked to it.  In
general we prefer to be frugal in our introduction of new ioctls,
especially when we have existing mechanisms via regions to support the
interactions.  The interface is designed to be flexible to the vendor
driver needs, partially thanks to it being a region.

   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)
And this is a good thing?

Well, I just meant that "having too much ioctls already" is not a good reason for not introducing new ones.

We can more easily deprecate and revise
region support than we can take back ioctls that have been previously

It belongs to uapi, how easily can we deprecate that?

I generally don't like the "let's create a new ioctl for that"
approach versus trying to fit something within the existing
architecture and convention.

(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
Propose how it could be implemented higher in the vfio stack to make it
device agnostic.

E.g doing it in vfio_device_fops instead of vfio_pci_ops?

2) Function duplicated with mdev and mdev can do even more
mdev also comes with a device lifecycle interface that doesn't really
make sense when a driver is only trying to partially mediate a single
physical device rather than multiplex a physical device into virtual

Yes, but that part could be decoupled out of mdev.

  mdev would also require vendor drivers to re-implement
much of vfio-pci for the direct access mechanisms.  Also, do we really
want users or management tools to decide between binding a device to
vfio-pci or a separate mdev driver to get this functionality.  We've
already been burnt trying to use mdev beyond its scope.

The problem is, if we had a device that support both SRIOV and mdev. Does this mean we need prepare two set of drivers?

     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
There's too much of mdev that doesn't make sense for this usage model,
this is why Yi's proposed generic mdev PCI wrapper is only a sample
driver.  I think we do not want to introduce user confusion regarding
which driver to use and there are outstanding non-singleton group
issues with mdev that don't seem worthwhile to resolve.

I agree, but I think what user want is a unified driver that works for both SRIOV and mdev. That's why trying to have a common way for doing mediation may make sense.


- The most straightforward way to support dirty page tracking is done by
IOMMU instead of device specific operations.
Of course, but it doesn't exist yet.  We're attempting to design the
dirty page tracking in a way that's mostly transparent for current mdev
drivers, would provide generic support for IOMMU-based dirty tracking,
and extensible to the inevitability of vendor driver tracking.  Thanks,


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