Re: [PATCH] vfio/iommufd: no hw_info for mediated devices

[Joao Martins <joao.m.martins@oracle.com>]

On 10/09/2024 18:40, Steve Sistare wrote:
> IOMMU_GET_HW_INFO fails for a vfio mediated device (aka mdev), because
> the devid is associated with kernel type IOMMUFD_OBJ_ACCESS, not
> IOMMUFD_OBJ_DEVICE.  Assume IOMMU_HW_INFO_TYPE_NONE and proceed.
> 
> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>

> ---
>  hw/vfio/iommufd.c | 6 ++++--
>  1 file changed, 4 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/vfio/iommufd.c b/hw/vfio/iommufd.c
> index cb7257b..d8928d4 100644
> --- a/hw/vfio/iommufd.c
> +++ b/hw/vfio/iommufd.c
> @@ -635,6 +635,7 @@ static void vfio_iommu_iommufd_class_init(ObjectClass *klass, void *data)
>  static bool hiod_iommufd_vfio_realize(HostIOMMUDevice *hiod, void *opaque,
>                                        Error **errp)
>  {
> +    Error *err = NULL;
>      VFIODevice *vdev = opaque;
>      HostIOMMUDeviceCaps *caps = &hiod->caps;
>      enum iommu_hw_info_type type;
> @@ -645,8 +646,9 @@ static bool hiod_iommufd_vfio_realize(HostIOMMUDevice *hiod, void *opaque,
>      hiod->agent = opaque;
>  
>      if (!iommufd_backend_get_device_info(vdev->iommufd, vdev->devid,
> -                                         &type, &data, sizeof(data), errp)) {
> -        return false;
> +                                         &type, &data, sizeof(data), &err)) {
> +        warn_report_err(err);
> +        type = IOMMU_HW_INFO_TYPE_NONE;
>      }
>  
>      hiod->name = g_strdup(vdev->name);

At least we aren't supposed to call realize() on an mdev.

See commit 9f17604195c ("vfio/iommufd: Don't initialize nor set a
HOST_IOMMU_DEVICE with mdev"). There's another refactor later on in commit
83a4d596a93 ("vfio/{iommufd, container}: Invoke HostIOMMUDevice::realize()
during attach_device()") where we also don't realize() when vbasedev->hiod is NULL.

Unless the stty mdevs aren't matching examples, then this shouldn't be possible
(I didn't have one with real hw behind to test).

So if you are hitting this codepath then vbasedev::mdev is false, which I don't
understand how it's possible?