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

[Jason Gunthorpe <jgg@nvidia.com>]

On Tue, Oct 20, 2026 at 11:41:57AM +0100, Ryan Roberts wrote:
> 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"; 

Well that's weird and a bit confusing.

> 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?

I don't know alot about your CLA, but this sounds reasonable. There
may be uses with containers to have the split exposed like this too.

Jason