Re: [PATCH] iommu: Fallback to default setting when def_domain_type() callback returns 0

[Robin Murphy]

On 2021-07-06 07:51, Kai-Heng Feng wrote:
> Commit 28b41e2c6aeb ("iommu: Move def_domain type check for untrusted
> device into core") not only moved the check for untrusted device to
> IOMMU core, it also introduced a behavioral change by returning
> def_domain_type() directly without checking its return value. That makes
> many devices no longer use the default IOMMU setting.
>
> So revert back to the old behavior which defaults to
> iommu_def_domain_type when driver callback returns 0.
>
> Fixes: 28b41e2c6aeb ("iommu: Move def_domain type check for untrusted device into core")

Are you sure about that? From that same commit:

@@ -1507,7 +1509,7 @@ static int iommu_alloc_default_domain(struct 
iommu_group *group,
        if (group->default_domain)
                return 0;

-       type = iommu_get_def_domain_type(dev);
+       type = iommu_get_def_domain_type(dev) ? : iommu_def_domain_type;

        return iommu_group_alloc_default_domain(dev->bus, group, type);
 }

AFAICS the other two callers should also handle 0 correctly. Have you 
seen a problem in practice?

Robin.

> Signed-off-by: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
> ---
>   drivers/iommu/iommu.c | 5 +++--
>   1 file changed, 3 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 5419c4b9f27a..faac4f795025 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1507,14 +1507,15 @@ EXPORT_SYMBOL_GPL(fsl_mc_device_group);
>   static int iommu_get_def_domain_type(struct device *dev)
>   {
>   	const struct iommu_ops *ops = dev->bus->iommu_ops;
> +	unsigned int type = 0;
>   
>   	if (dev_is_pci(dev) && to_pci_dev(dev)->untrusted)
>   		return IOMMU_DOMAIN_DMA;
>   
>   	if (ops->def_domain_type)
> -		return ops->def_domain_type(dev);
> +		type = ops->def_domain_type(dev);
>   
> -	return 0;
> +	return (type == 0) ? iommu_def_domain_type : type;
>   }
>   
>   static int iommu_group_alloc_default_domain(struct bus_type *bus,