Re: [RFC PATCH kernel] vfio: Skip checking for IOMMU_CAP_CACHE_COHERENCY on POWER and more

On Fri, Jul 01, 2022 at 04:17:51PM +1000, Alexey Kardashevskiy wrote:
> VFIO on POWER does not implement iommu_ops and therefore iommu_capable()
> always returns false and __iommu_group_alloc_blocking_domain() always
> fails.
> 
> iommu_group_claim_dma_owner() in setting container fails for the same
> reason - it cannot allocate a domain.
> 
> This skips the check for platforms supporting VFIO without implementing
> iommu_ops which to my best knowledge is POWER only.
> 
> This also allows setting container in absence of iommu_ops.
> 
> Fixes: 70693f470848 ("vfio: Set DMA ownership for VFIO devices")
> Fixes: e8ae0e140c05 ("vfio: Require that devices support DMA cache coherence")
> Signed-off-by: Alexey Kardashevskiy <[email protected]>
> ---
> 
> Not quite sure what the proper small fix is and implementing iommu_ops
> on POWER is not going to happen any time soon or ever :-/
> 
> ---
>  drivers/vfio/vfio.c | 13 +++++++------
>  1 file changed, 7 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index 61e71c1154be..71408ab26cd0 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -605,7 +605,8 @@ int vfio_register_group_dev(struct vfio_device *device)
>  	 * VFIO always sets IOMMU_CACHE because we offer no way for userspace to
>  	 * restore cache coherency.
>  	 */
> -	if (!iommu_capable(device->dev->bus, IOMMU_CAP_CACHE_COHERENCY))
> +	if (device->dev->bus->iommu_ops &&
> +	    !iommu_capable(device->dev->bus, IOMMU_CAP_CACHE_COHERENCY))
>  		return -EINVAL;

This change should be guarded by some
IS_ENABLED(CONFIG_VFIO_IOMMU_SPAPR_TCE)

We want to do the this check here on every other
configuration. Rejecting null iommu_ops is actually a desired side
effect.

>  	return __vfio_register_dev(device,
> @@ -934,7 +935,7 @@ static void __vfio_group_unset_container(struct vfio_group *group)
>  		driver->ops->detach_group(container->iommu_data,
>  					  group->iommu_group);
>  
> -	if (group->type = VFIO_IOMMU)
> +	if (group->type = VFIO_IOMMU && iommu_group_dma_owner_claimed(group->iommu_group))
>  		iommu_group_release_dma_owner(group->iommu_group);
>  
>  	group->container = NULL;
> @@ -1010,9 +1011,8 @@ static int vfio_group_set_container(struct vfio_group *group, int container_fd)
>  	}
>  
>  	if (group->type = VFIO_IOMMU) {
> -		ret = iommu_group_claim_dma_owner(group->iommu_group, f.file);
> -		if (ret)
> -			goto unlock_out;
> +		if (iommu_group_claim_dma_owner(group->iommu_group, f.file))
> +			pr_warn("Failed to claim DMA owner");

We certainly cannot ignore this. As my other email you should make
this succeed inside the iommu subsystem even though the ops are null.

Thanks,
Jason