Re: [PATCH v4 28/41] vfio/iommufd: Implement the iommufd backend

[Jason Gunthorpe]

On Wed, Nov 08, 2023 at 07:16:52AM +0000, Duan, Zhenzhong wrote:

> >> +    ret = iommufd_backend_alloc_hwpt(iommufd, vbasedev->devid,
> >> +                                     container->ioas_id, &hwpt_id);
> >> +
> >> +    if (ret) {
> >> +        error_setg_errno(errp, errno, "error alloc shadow hwpt");
> >> +        return ret;
> >> +    }
> >
> >The above alloc_hwpt fails for mdevs (at least, it fails for me attempting 
> >to use
> >iommufd backend with vfio-ccw and vfio-ap on s390).  The ioctl is failing in 
> >the
> >kernel because it can't find an IOMMUFD_OBJ_DEVICE.
> >
> >AFAIU that's because the mdevs are meant to instead use kernel access via
> >vfio_iommufd_emulated_attach_ioas, not hwpt.  That's how mdevs behave when
> >looking at the kernel vfio compat container.
> >
> >As a test, I was able to get vfio-ccw and vfio-ap working using the iommufd
> >backend by just skipping this alloc_hwpt above and instead passing container-
> >>ioas_id into the iommufd_cdev_attach_hwpt below.  That triggers the
> >vfio_iommufd_emulated_attach_ioas call in the kernel.
> 
> Thanks for help test and investigation.
> I was only focusing on real device and missed the mdev particularity, sorry.
> You are right, there is no hwpt support for mdev, not even an emulated hwpt.
> I'll digging into this and see how to distinguish mdev with real device in
> this low level function.

I was expecting that hwpt manipulation would be done exclusively
inside the device-specific vIOMMU userspace driver. Generic code paths
that don't have that knowledge should use the IOAS for everything

Jason