Linux Virtualization - Jul 2019 - [RFC v2] vhost: introduce mdev based hardware vhost backend

On Thu, Jul 04, 2019 at 12:31:48PM +0800, Jason Wang
wrote:
> On 2019/7/3 ??9:08, Tiwei Bie wrote:
> > On Wed, Jul 03, 2019 at 08:16:23PM +0800, Jason Wang wrote:
> > > On 2019/7/3 ??7:52, Tiwei Bie wrote:
> > > > On Wed, Jul 03, 2019 at 06:09:51PM +0800, Jason Wang wrote:
> > > > > On 2019/7/3 ??5:13, Tiwei Bie wrote:
> > > > > > Details about this can be found here:
> > > > > > 
> > > > > > https://lwn.net/Articles/750770/
> > > > > > 
> > > > > > What's new in this version
> > > > > > =========================> > > > >
>
> > > > > > A new VFIO device type is introduced - vfio-vhost.
This addressed
> > > > > > some comments from
here:https://patchwork.ozlabs.org/cover/984763/
> > > > > > 
> > > > > > Below is the updated device interface:
> > > > > > 
> > > > > > Currently, there are two regions of this device:
1) CONFIG_REGION
> > > > > > (VFIO_VHOST_CONFIG_REGION_INDEX), which can be
used to setup the
> > > > > > device; 2) NOTIFY_REGION
(VFIO_VHOST_NOTIFY_REGION_INDEX), which
> > > > > > can be used to notify the device.
> > > > > > 
> > > > > > 1. CONFIG_REGION
> > > > > > 
> > > > > > The region described by CONFIG_REGION is the main
control interface.
> > > > > > Messages will be written to or read from this
region.
> > > > > > 
> > > > > > The message type is determined by the `request`
field in message
> > > > > > header. The message size is encoded in the message
header too.
> > > > > > The message format looks like this:
> > > > > > 
> > > > > > struct vhost_vfio_op {
> > > > > > 	__u64 request;
> > > > > > 	__u32 flags;
> > > > > > 	/* Flag values: */
> > > > > >     #define VHOST_VFIO_NEED_REPLY 0x1 /* Whether
need reply */
> > > > > > 	__u32 size;
> > > > > > 	union {
> > > > > > 		__u64 u64;
> > > > > > 		struct vhost_vring_state state;
> > > > > > 		struct vhost_vring_addr addr;
> > > > > > 	} payload;
> > > > > > };
> > > > > > 
> > > > > > The existing vhost-kernel ioctl cmds are reused as
the message
> > > > > > requests in above structure.
> > > > > Still a comments like V1. What's the advantage of
inventing a new protocol?
> > > > I'm trying to make it work in VFIO's way..
> > > > 
> > > > > I believe either of the following should be better:
> > > > > 
> > > > > - using vhost ioctl,? we can start from
SET_VRING_KICK/SET_VRING_CALL and
> > > > > extend it with e.g notify region. The advantages is
that all exist userspace
> > > > > program could be reused without modification (or
minimal modification). And
> > > > > vhost API hides lots of details that is not necessary
to be understood by
> > > > > application (e.g in the case of container).
> > > > Do you mean reusing vhost's ioctl on VFIO device fd
directly,
> > > > or introducing another mdev driver (i.e. vhost_mdev instead
of
> > > > using the existing vfio_mdev) for mdev device?
> > > Can we simply add them into ioctl of mdev_parent_ops?
> > Right, either way, these ioctls have to be and just need to be
> > added in the ioctl of the mdev_parent_ops. But another thing we
> > also need to consider is that which file descriptor the userspace
> > will do the ioctl() on. So I'm wondering do you mean let the
> > userspace do the ioctl() on the VFIO device fd of the mdev
> > device?
> > 
> 
> Yes.
Got it! I'm not sure what's Alex opinion on this. If we all
agree with this, I can do it in this way.
> Is there any other way btw?
Just a quick thought.. Maybe totally a bad idea. I was thinking
whether it would be odd to do non-VFIO's ioctls on VFIO's device
fd. So I was wondering whether it's possible to allow binding
another mdev driver (e.g. vhost_mdev) to the supported mdev
devices. The new mdev driver, vhost_mdev, can provide similar
ways to let userspace open the mdev device and do the vhost ioctls
on it. To distinguish with the vfio_mdev compatible mdev devices,
the device API of the new vhost_mdev compatible mdev devices
might be e.g. "vhost-net" for net?

So in VFIO case, the device will be for passthru directly. And
in VHOST case, the device can be used to accelerate the existing
virtualized devices.

How do you think?

Thanks,
Tiwei
> 
> Thanks
>

On 2019/7/4 ??2:21, Tiwei Bie wrote:
> On Thu, Jul 04, 2019 at 12:31:48PM +0800, Jason Wang wrote:
>> On 2019/7/3 ??9:08, Tiwei Bie wrote:
>>> On Wed, Jul 03, 2019 at 08:16:23PM +0800, Jason Wang wrote:
>>>> On 2019/7/3 ??7:52, Tiwei Bie wrote:
>>>>> On Wed, Jul 03, 2019 at 06:09:51PM +0800, Jason Wang wrote:
>>>>>> On 2019/7/3 ??5:13, Tiwei Bie wrote:
>>>>>>> Details about this can be found here:
>>>>>>>
>>>>>>> https://lwn.net/Articles/750770/
>>>>>>>
>>>>>>> What's new in this version
>>>>>>>
=========================>>>>>>>
>>>>>>> A new VFIO device type is introduced - vfio-vhost.
This addressed
>>>>>>> some comments from
here:https://patchwork.ozlabs.org/cover/984763/
>>>>>>>
>>>>>>> Below is the updated device interface:
>>>>>>>
>>>>>>> Currently, there are two regions of this device: 1)
CONFIG_REGION
>>>>>>> (VFIO_VHOST_CONFIG_REGION_INDEX), which can be used
to setup the
>>>>>>> device; 2) NOTIFY_REGION
(VFIO_VHOST_NOTIFY_REGION_INDEX), which
>>>>>>> can be used to notify the device.
>>>>>>>
>>>>>>> 1. CONFIG_REGION
>>>>>>>
>>>>>>> The region described by CONFIG_REGION is the main
control interface.
>>>>>>> Messages will be written to or read from this
region.
>>>>>>>
>>>>>>> The message type is determined by the `request`
field in message
>>>>>>> header. The message size is encoded in the message
header too.
>>>>>>> The message format looks like this:
>>>>>>>
>>>>>>> struct vhost_vfio_op {
>>>>>>> 	__u64 request;
>>>>>>> 	__u32 flags;
>>>>>>> 	/* Flag values: */
>>>>>>>      #define VHOST_VFIO_NEED_REPLY 0x1 /* Whether
need reply */
>>>>>>> 	__u32 size;
>>>>>>> 	union {
>>>>>>> 		__u64 u64;
>>>>>>> 		struct vhost_vring_state state;
>>>>>>> 		struct vhost_vring_addr addr;
>>>>>>> 	} payload;
>>>>>>> };
>>>>>>>
>>>>>>> The existing vhost-kernel ioctl cmds are reused as
the message
>>>>>>> requests in above structure.
>>>>>> Still a comments like V1. What's the advantage of
inventing a new protocol?
>>>>> I'm trying to make it work in VFIO's way..
>>>>>
>>>>>> I believe either of the following should be better:
>>>>>>
>>>>>> - using vhost ioctl,? we can start from
SET_VRING_KICK/SET_VRING_CALL and
>>>>>> extend it with e.g notify region. The advantages is
that all exist userspace
>>>>>> program could be reused without modification (or
minimal modification). And
>>>>>> vhost API hides lots of details that is not necessary
to be understood by
>>>>>> application (e.g in the case of container).
>>>>> Do you mean reusing vhost's ioctl on VFIO device fd
directly,
>>>>> or introducing another mdev driver (i.e. vhost_mdev instead
of
>>>>> using the existing vfio_mdev) for mdev device?
>>>> Can we simply add them into ioctl of mdev_parent_ops?
>>> Right, either way, these ioctls have to be and just need to be
>>> added in the ioctl of the mdev_parent_ops. But another thing we
>>> also need to consider is that which file descriptor the userspace
>>> will do the ioctl() on. So I'm wondering do you mean let the
>>> userspace do the ioctl() on the VFIO device fd of the mdev
>>> device?
>>>
>> Yes.
> Got it! I'm not sure what's Alex opinion on this. If we all
> agree with this, I can do it in this way.
>
>> Is there any other way btw?
> Just a quick thought.. Maybe totally a bad idea.

It's not for sure :)

>   I was thinking
> whether it would be odd to do non-VFIO's ioctls on VFIO's device
> fd. So I was wondering whether it's possible to allow binding
> another mdev driver (e.g. vhost_mdev) to the supported mdev
> devices. The new mdev driver, vhost_mdev, can provide similar
> ways to let userspace open the mdev device and do the vhost ioctls
> on it. To distinguish with the vfio_mdev compatible mdev devices,
> the device API of the new vhost_mdev compatible mdev devices
> might be e.g. "vhost-net" for net?
>
> So in VFIO case, the device will be for passthru directly. And
> in VHOST case, the device can be used to accelerate the existing
> virtualized devices.
>
> How do you think?

If my understanding is correct, there will be no VFIO ioctl if we go for 
vhost_mdev?

Thanks

>
> Thanks,
> Tiwei
>> Thanks
>>

On Thu, Jul 04, 2019 at 02:35:20PM +0800, Jason Wang
wrote:
> On 2019/7/4 ??2:21, Tiwei Bie wrote:
> > On Thu, Jul 04, 2019 at 12:31:48PM +0800, Jason Wang wrote:
> > > On 2019/7/3 ??9:08, Tiwei Bie wrote:
> > > > On Wed, Jul 03, 2019 at 08:16:23PM +0800, Jason Wang wrote:
> > > > > On 2019/7/3 ??7:52, Tiwei Bie wrote:
> > > > > > On Wed, Jul 03, 2019 at 06:09:51PM +0800, Jason
Wang wrote:
> > > > > > > On 2019/7/3 ??5:13, Tiwei Bie wrote:
> > > > > > > > Details about this can be found here:
> > > > > > > > 
> > > > > > > > https://lwn.net/Articles/750770/
> > > > > > > > 
> > > > > > > > What's new in this version
> > > > > > > > =========================> > >
> > > > >
> > > > > > > > A new VFIO device type is introduced -
vfio-vhost. This addressed
> > > > > > > > some comments from
here:https://patchwork.ozlabs.org/cover/984763/
> > > > > > > > 
> > > > > > > > Below is the updated device interface:
> > > > > > > > 
> > > > > > > > Currently, there are two regions of this
device: 1) CONFIG_REGION
> > > > > > > > (VFIO_VHOST_CONFIG_REGION_INDEX), which
can be used to setup the
> > > > > > > > device; 2) NOTIFY_REGION
(VFIO_VHOST_NOTIFY_REGION_INDEX), which
> > > > > > > > can be used to notify the device.
> > > > > > > > 
> > > > > > > > 1. CONFIG_REGION
> > > > > > > > 
> > > > > > > > The region described by CONFIG_REGION is
the main control interface.
> > > > > > > > Messages will be written to or read from
this region.
> > > > > > > > 
> > > > > > > > The message type is determined by the
`request` field in message
> > > > > > > > header. The message size is encoded in
the message header too.
> > > > > > > > The message format looks like this:
> > > > > > > > 
> > > > > > > > struct vhost_vfio_op {
> > > > > > > > 	__u64 request;
> > > > > > > > 	__u32 flags;
> > > > > > > > 	/* Flag values: */
> > > > > > > >      #define VHOST_VFIO_NEED_REPLY 0x1
/* Whether need reply */
> > > > > > > > 	__u32 size;
> > > > > > > > 	union {
> > > > > > > > 		__u64 u64;
> > > > > > > > 		struct vhost_vring_state state;
> > > > > > > > 		struct vhost_vring_addr addr;
> > > > > > > > 	} payload;
> > > > > > > > };
> > > > > > > > 
> > > > > > > > The existing vhost-kernel ioctl cmds are
reused as the message
> > > > > > > > requests in above structure.
> > > > > > > Still a comments like V1. What's the
advantage of inventing a new protocol?
> > > > > > I'm trying to make it work in VFIO's way..
> > > > > > 
> > > > > > > I believe either of the following should be
better:
> > > > > > > 
> > > > > > > - using vhost ioctl,? we can start from
SET_VRING_KICK/SET_VRING_CALL and
> > > > > > > extend it with e.g notify region. The
advantages is that all exist userspace
> > > > > > > program could be reused without modification
(or minimal modification). And
> > > > > > > vhost API hides lots of details that is not
necessary to be understood by
> > > > > > > application (e.g in the case of container).
> > > > > > Do you mean reusing vhost's ioctl on VFIO
device fd directly,
> > > > > > or introducing another mdev driver (i.e.
vhost_mdev instead of
> > > > > > using the existing vfio_mdev) for mdev device?
> > > > > Can we simply add them into ioctl of mdev_parent_ops?
> > > > Right, either way, these ioctls have to be and just need to
be
> > > > added in the ioctl of the mdev_parent_ops. But another thing
we
> > > > also need to consider is that which file descriptor the
userspace
> > > > will do the ioctl() on. So I'm wondering do you mean let
the
> > > > userspace do the ioctl() on the VFIO device fd of the mdev
> > > > device?
> > > > 
> > > Yes.
> > Got it! I'm not sure what's Alex opinion on this. If we all
> > agree with this, I can do it in this way.
> > 
> > > Is there any other way btw?
> > Just a quick thought.. Maybe totally a bad idea.
> 
> 
> It's not for sure :)
Thanks!
> 
> 
> >   I was thinking
> > whether it would be odd to do non-VFIO's ioctls on VFIO's
device
> > fd. So I was wondering whether it's possible to allow binding
> > another mdev driver (e.g. vhost_mdev) to the supported mdev
> > devices. The new mdev driver, vhost_mdev, can provide similar
> > ways to let userspace open the mdev device and do the vhost ioctls
> > on it. To distinguish with the vfio_mdev compatible mdev devices,
> > the device API of the new vhost_mdev compatible mdev devices
> > might be e.g. "vhost-net" for net?
> > 
> > So in VFIO case, the device will be for passthru directly. And
> > in VHOST case, the device can be used to accelerate the existing
> > virtualized devices.
> > 
> > How do you think?
> 
> 
> If my understanding is correct, there will be no VFIO ioctl if we go for
> vhost_mdev?
Yeah, exactly. If we go for vhost_mdev, we may have some vhost nodes
in /dev similar to what /dev/vfio/* does to handle the $UUID and open
the device (e.g. similar to VFIO_GROUP_GET_DEVICE_FD in VFIO). And
to setup the device, we can try to reuse the ioctls of the existing
kernel vhost as much as possible.

Thanks,
Tiwei
> 
> Thanks
> 
> 
> > 
> > Thanks,
> > Tiwei
> > > Thanks
> > >

On Thu, 4 Jul 2019 14:21:34 +0800
Tiwei Bie <tiwei.bie at intel.com> wrote:
> On Thu, Jul 04, 2019 at 12:31:48PM +0800, Jason Wang wrote:
> > On 2019/7/3 ??9:08, Tiwei Bie wrote:  
> > > On Wed, Jul 03, 2019 at 08:16:23PM +0800, Jason Wang wrote:  
> > > > On 2019/7/3 ??7:52, Tiwei Bie wrote:  
> > > > > On Wed, Jul 03, 2019 at 06:09:51PM +0800, Jason Wang
wrote:
> > > > > > On 2019/7/3 ??5:13, Tiwei Bie wrote:  
> > > > > > > Details about this can be found here:
> > > > > > > 
> > > > > > > https://lwn.net/Articles/750770/
> > > > > > > 
> > > > > > > What's new in this version
> > > > > > > =========================> > > >
> > >
> > > > > > > A new VFIO device type is introduced -
vfio-vhost. This addressed
> > > > > > > some comments from
here:https://patchwork.ozlabs.org/cover/984763/
> > > > > > > 
> > > > > > > Below is the updated device interface:
> > > > > > > 
> > > > > > > Currently, there are two regions of this
device: 1) CONFIG_REGION
> > > > > > > (VFIO_VHOST_CONFIG_REGION_INDEX), which can
be used to setup the
> > > > > > > device; 2) NOTIFY_REGION
(VFIO_VHOST_NOTIFY_REGION_INDEX), which
> > > > > > > can be used to notify the device.
> > > > > > > 
> > > > > > > 1. CONFIG_REGION
> > > > > > > 
> > > > > > > The region described by CONFIG_REGION is the
main control interface.
> > > > > > > Messages will be written to or read from this
region.
> > > > > > > 
> > > > > > > The message type is determined by the
`request` field in message
> > > > > > > header. The message size is encoded in the
message header too.
> > > > > > > The message format looks like this:
> > > > > > > 
> > > > > > > struct vhost_vfio_op {
> > > > > > > 	__u64 request;
> > > > > > > 	__u32 flags;
> > > > > > > 	/* Flag values: */
> > > > > > >     #define VHOST_VFIO_NEED_REPLY 0x1 /*
Whether need reply */
> > > > > > > 	__u32 size;
> > > > > > > 	union {
> > > > > > > 		__u64 u64;
> > > > > > > 		struct vhost_vring_state state;
> > > > > > > 		struct vhost_vring_addr addr;
> > > > > > > 	} payload;
> > > > > > > };
> > > > > > > 
> > > > > > > The existing vhost-kernel ioctl cmds are
reused as the message
> > > > > > > requests in above structure.  
> > > > > > Still a comments like V1. What's the advantage
of inventing a new protocol?
> > > > > I'm trying to make it work in VFIO's way..
> > > > >   
> > > > > > I believe either of the following should be
better:
> > > > > > 
> > > > > > - using vhost ioctl,? we can start from
SET_VRING_KICK/SET_VRING_CALL and
> > > > > > extend it with e.g notify region. The advantages
is that all exist userspace
> > > > > > program could be reused without modification (or
minimal modification). And
> > > > > > vhost API hides lots of details that is not
necessary to be understood by
> > > > > > application (e.g in the case of container).  
> > > > > Do you mean reusing vhost's ioctl on VFIO device fd
directly,
> > > > > or introducing another mdev driver (i.e. vhost_mdev
instead of
> > > > > using the existing vfio_mdev) for mdev device?  
> > > > Can we simply add them into ioctl of mdev_parent_ops?  
> > > Right, either way, these ioctls have to be and just need to be
> > > added in the ioctl of the mdev_parent_ops. But another thing we
> > > also need to consider is that which file descriptor the userspace
> > > will do the ioctl() on. So I'm wondering do you mean let the
> > > userspace do the ioctl() on the VFIO device fd of the mdev
> > > device?
> > >   
> > 
> > Yes.  
> 
> Got it! I'm not sure what's Alex opinion on this. If we all
> agree with this, I can do it in this way.
> 
> > Is there any other way btw?  
> 
> Just a quick thought.. Maybe totally a bad idea. I was thinking
> whether it would be odd to do non-VFIO's ioctls on VFIO's device
> fd. So I was wondering whether it's possible to allow binding
> another mdev driver (e.g. vhost_mdev) to the supported mdev
> devices. The new mdev driver, vhost_mdev, can provide similar
> ways to let userspace open the mdev device and do the vhost ioctls
> on it. To distinguish with the vfio_mdev compatible mdev devices,
> the device API of the new vhost_mdev compatible mdev devices
> might be e.g. "vhost-net" for net?
> 
> So in VFIO case, the device will be for passthru directly. And
> in VHOST case, the device can be used to accelerate the existing
> virtualized devices.
> 
> How do you think?
VFIO really can't prevent vendor specific ioctls on the device file
descriptor for mdevs, but a) we'd want to be sure the ioctl address
space can't collide with ioctls we'd use for vfio defined purposes and
b) maybe the VFIO user API isn't what you want in the first place if
you intend to mostly/entirely ignore the defined ioctl set and replace
them with your own.  In the case of the latter, you're also not getting
the advantages of the existing VFIO userspace code, so why expose a
VFIO device at all.

The mdev interface does provide a general interface for creating and
managing virtual devices, vfio-mdev is just one driver on the mdev
bus.  Parav (Mellanox) has been doing work on mdev-core to help clean
out vfio-isms from the interface, aiui, with the intent of implementing
another mdev bus driver for using the devices within the kernel.  It
seems like this vhost-mdev driver might be similar, using mdev but not
necessarily vfio-mdev to expose devices.  Thanks,

Alex

On Fri, Jul 05, 2019 at 08:49:46AM -0600, Alex Williamson
wrote:
> On Thu, 4 Jul 2019 14:21:34 +0800
> Tiwei Bie <tiwei.bie at intel.com> wrote:
> > On Thu, Jul 04, 2019 at 12:31:48PM +0800, Jason Wang wrote:
> > > On 2019/7/3 ??9:08, Tiwei Bie wrote:  
> > > > On Wed, Jul 03, 2019 at 08:16:23PM +0800, Jason Wang wrote:
> > > > > On 2019/7/3 ??7:52, Tiwei Bie wrote:  
> > > > > > On Wed, Jul 03, 2019 at 06:09:51PM +0800, Jason
Wang wrote:
> > > > > > > On 2019/7/3 ??5:13, Tiwei Bie wrote:  
> > > > > > > > Details about this can be found here:
> > > > > > > > 
> > > > > > > > https://lwn.net/Articles/750770/
> > > > > > > > 
> > > > > > > > What's new in this version
> > > > > > > > =========================> > >
> > > > >
> > > > > > > > A new VFIO device type is introduced -
vfio-vhost. This addressed
> > > > > > > > some comments from
here:https://patchwork.ozlabs.org/cover/984763/
> > > > > > > > 
> > > > > > > > Below is the updated device interface:
> > > > > > > > 
> > > > > > > > Currently, there are two regions of this
device: 1) CONFIG_REGION
> > > > > > > > (VFIO_VHOST_CONFIG_REGION_INDEX), which
can be used to setup the
> > > > > > > > device; 2) NOTIFY_REGION
(VFIO_VHOST_NOTIFY_REGION_INDEX), which
> > > > > > > > can be used to notify the device.
> > > > > > > > 
> > > > > > > > 1. CONFIG_REGION
> > > > > > > > 
> > > > > > > > The region described by CONFIG_REGION is
the main control interface.
> > > > > > > > Messages will be written to or read from
this region.
> > > > > > > > 
> > > > > > > > The message type is determined by the
`request` field in message
> > > > > > > > header. The message size is encoded in
the message header too.
> > > > > > > > The message format looks like this:
> > > > > > > > 
> > > > > > > > struct vhost_vfio_op {
> > > > > > > > 	__u64 request;
> > > > > > > > 	__u32 flags;
> > > > > > > > 	/* Flag values: */
> > > > > > > >     #define VHOST_VFIO_NEED_REPLY 0x1 /*
Whether need reply */
> > > > > > > > 	__u32 size;
> > > > > > > > 	union {
> > > > > > > > 		__u64 u64;
> > > > > > > > 		struct vhost_vring_state state;
> > > > > > > > 		struct vhost_vring_addr addr;
> > > > > > > > 	} payload;
> > > > > > > > };
> > > > > > > > 
> > > > > > > > The existing vhost-kernel ioctl cmds are
reused as the message
> > > > > > > > requests in above structure.  
> > > > > > > Still a comments like V1. What's the
advantage of inventing a new protocol?
> > > > > > I'm trying to make it work in VFIO's way..
> > > > > >   
> > > > > > > I believe either of the following should be
better:
> > > > > > > 
> > > > > > > - using vhost ioctl,? we can start from
SET_VRING_KICK/SET_VRING_CALL and
> > > > > > > extend it with e.g notify region. The
advantages is that all exist userspace
> > > > > > > program could be reused without modification
(or minimal modification). And
> > > > > > > vhost API hides lots of details that is not
necessary to be understood by
> > > > > > > application (e.g in the case of container).  
> > > > > > Do you mean reusing vhost's ioctl on VFIO
device fd directly,
> > > > > > or introducing another mdev driver (i.e.
vhost_mdev instead of
> > > > > > using the existing vfio_mdev) for mdev device?  
> > > > > Can we simply add them into ioctl of mdev_parent_ops?  
> > > > Right, either way, these ioctls have to be and just need to
be
> > > > added in the ioctl of the mdev_parent_ops. But another thing
we
> > > > also need to consider is that which file descriptor the
userspace
> > > > will do the ioctl() on. So I'm wondering do you mean let
the
> > > > userspace do the ioctl() on the VFIO device fd of the mdev
> > > > device?
> > > >   
> > > 
> > > Yes.  
> > 
> > Got it! I'm not sure what's Alex opinion on this. If we all
> > agree with this, I can do it in this way.
> > 
> > > Is there any other way btw?  
> > 
> > Just a quick thought.. Maybe totally a bad idea. I was thinking
> > whether it would be odd to do non-VFIO's ioctls on VFIO's
device
> > fd. So I was wondering whether it's possible to allow binding
> > another mdev driver (e.g. vhost_mdev) to the supported mdev
> > devices. The new mdev driver, vhost_mdev, can provide similar
> > ways to let userspace open the mdev device and do the vhost ioctls
> > on it. To distinguish with the vfio_mdev compatible mdev devices,
> > the device API of the new vhost_mdev compatible mdev devices
> > might be e.g. "vhost-net" for net?
> > 
> > So in VFIO case, the device will be for passthru directly. And
> > in VHOST case, the device can be used to accelerate the existing
> > virtualized devices.
> > 
> > How do you think?
> 
> VFIO really can't prevent vendor specific ioctls on the device file
> descriptor for mdevs, but a) we'd want to be sure the ioctl address
> space can't collide with ioctls we'd use for vfio defined purposes
and
> b) maybe the VFIO user API isn't what you want in the first place if
> you intend to mostly/entirely ignore the defined ioctl set and replace
> them with your own.  In the case of the latter, you're also not getting
> the advantages of the existing VFIO userspace code, so why expose a
> VFIO device at all.
Yeah, I totally agree.
> 
> The mdev interface does provide a general interface for creating and
> managing virtual devices, vfio-mdev is just one driver on the mdev
> bus.  Parav (Mellanox) has been doing work on mdev-core to help clean
> out vfio-isms from the interface, aiui, with the intent of implementing
> another mdev bus driver for using the devices within the kernel.
Great to know this! I found below series after some searching:

https://lkml.org/lkml/2019/3/8/821

In above series, the new mlx5_core mdev driver will do the probe
by calling mlx5_get_core_dev() first on the parent device of the
mdev device. In vhost_mdev, maybe we can also keep track of all
the compatible mdev devices and use this info to do the probe.
But we also need a way to allow vfio_mdev driver to distinguish
and reject the incompatible mdev devices.
> It
> seems like this vhost-mdev driver might be similar, using mdev but not
> necessarily vfio-mdev to expose devices.  Thanks,
Yeah, I also think so!

Thanks!
Tiwei
> 
> Alex