Re: [PATCH v3 4/6] iommu: Move IOMMU pagesize check to attach_device

[Lu Baolu <baolu.lu@linux.intel.com>]

On 2021/10/20 0:37, Sven Peter via iommu wrote:
> The iova allocator is capable of handling any granularity which is a power
> of two. Remove the much stronger condition that the granularity must be
> smaller or equal to the CPU page size from a BUG_ON there.
> Instead, check this condition during __iommu_attach_device and fail
> gracefully.
> 
> Signed-off-by: Sven Peter<sven@svenpeter.dev>
> ---
>   drivers/iommu/iommu.c | 35 ++++++++++++++++++++++++++++++++---
>   drivers/iommu/iova.c  |  7 ++++---
>   include/linux/iommu.h |  5 +++++
>   3 files changed, 41 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index dd7863e453a5..28896739964b 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -80,6 +80,8 @@ static struct iommu_domain *__iommu_domain_alloc(struct bus_type *bus,
>   						 unsigned type);
>   static int __iommu_attach_device(struct iommu_domain *domain,
>   				 struct device *dev);
> +static void __iommu_detach_device(struct iommu_domain *domain,
> +				  struct device *dev);
>   static int __iommu_attach_group(struct iommu_domain *domain,
>   				struct iommu_group *group);
>   static void __iommu_detach_group(struct iommu_domain *domain,
> @@ -1974,6 +1976,19 @@ void iommu_domain_free(struct iommu_domain *domain)
>   }
>   EXPORT_SYMBOL_GPL(iommu_domain_free);
>   
> +static int iommu_check_page_size(struct iommu_domain *domain)
> +{
> +	if (!iommu_is_paging_domain(domain))
> +		return 0;
> +
> +	if (!(domain->pgsize_bitmap & (PAGE_SIZE | (PAGE_SIZE - 1)))) {
> +		pr_warn("IOMMU pages cannot exactly represent CPU pages.\n");
> +		return -EFAULT;
> +	}
> +
> +	return 0;
> +}
> +
>   static int __iommu_attach_device(struct iommu_domain *domain,
>   				 struct device *dev)
>   {
> @@ -1983,9 +1998,23 @@ static int __iommu_attach_device(struct iommu_domain *domain,
>   		return -ENODEV;
>   
>   	ret = domain->ops->attach_dev(domain, dev);
> -	if (!ret)
> -		trace_attach_device_to_domain(dev);
> -	return ret;
> +	if (ret)
> +		return ret;
> +
> +	/*
> +	 * Check that CPU pages can be represented by the IOVA granularity.
> +	 * This has to be done after ops->attach_dev since many IOMMU drivers
> +	 * only limit domain->pgsize_bitmap after having attached the first
> +	 * device.
> +	 */
> +	ret = iommu_check_page_size(domain);
> +	if (ret) {
> +		__iommu_detach_device(domain, dev);
> +		return ret;
> +	}

It looks odd. __iommu_attach_device() attaches an I/O page table for a
device. How does it relate to CPU pages? Why is it a failure case if CPU
page size is not covered?

Best regards,
baolu