Re: [RFC PATCH v2 15/15] misc/arm-cla: Add vfio-mdev user interface

[Ryan Roberts <ryan.roberts@arm.com>]

On 16/10/2026 15:42, Jason Gunthorpe wrote:
> On Fri, Oct 16, 2026 at 03:22:00PM +0100, Jean-Philippe Brucker wrote:
>> On Thu, 15 Oct 2026 at 17:41, Jason Gunthorpe <jgg@nvidia.com> wrote:
>>> I think I said this last time, please don't use vfio mdev for
>>> anything. There is nothing mediated about this.
>>
>> We don't multiplex a device between guests, but the host driver must
>> control part of the device registers while the remainder is assigned to
>> a guest. The host configures accelerator access, stage-2 translation,
>> handles CPUHP and context save/restore. So we can't bind the device to
>> vfio-platform.
> 
> That's fine, you can make a proper vfio-cla driver.

Hi Jason,

Thanks for taking a look at this! I've been chewing on this comment and trying
to educate myself deeper on the subject. I have a proposal...

Currently we create a struct device per CLA device instance. But multiple CLA
device instances cooperate within a domain and the domain is the "unit of
isolation"; we can't have some CLAs from a domain assigned to a VM, while others
from the same domain are used by the host. Therefore it didn't feel like the
correct model to unbind one of these devices from the native driver and rebind
it to a cla-vfio driver. mdev seemed to solve this problem nicely for us.

But looking into it in more detail, I guess something like this might be a bit
more standard:

 - Split the existing CLA driver into three components:
    - cla-core: Hardware discovery, domain configuration, struct device
      registration and CPU hotplug
    - cla-native driver: Provides the user space accelerator interfaces
    - cla-vfio driver: Enables domains to be assigned to virtual machines
 - Each domain has a struct device (created by the core)
    - An individual CLA is not represented by a struct device
 - Either cla-native or cla-vfio can then bind to a domain device
    - cla-native exposes a char device per domain, per accelerator class
    - cla-vfio

For VFIO, I guess we could use one VFIO region per CLA, which would allow the
VMM to construct a device tree. But it would need to additionally determine
which CPU each CLA is bound to; perhaps that's a sysfs interface that cla-core
could expose?

Does this feel like it's going in the right direction?

Thanks,
Ryan

> 
> Jason