Subject: Re: [PATCH v6 02/25] iommu: Add IOMMU_DOMAIN_PLATFORM

From: Baolu Lu <[email-masked]>
Date: Sat, 12 Aug 2023 09:36:33 +0800

On 2023/8/3 8:07, Jason Gunthorpe wrote:
> This is used when the iommu driver is taking control of the dma_ops,
> currently only on S390 and power spapr. It is designed to preserve the
> original ops->detach_dev() semantic that these S390 was built around.
> 
> Provide an opaque domain type and a 'default_domain' ops value that allows
> the driver to trivially force any single domain as the default domain.
> 
> Reviewed-by: Lu Baolu <[email-masked]>
> Signed-off-by: Jason Gunthorpe <[email-masked]>
> ---
>   drivers/iommu/iommu.c | 14 +++++++++++++-
>   include/linux/iommu.h |  6 ++++++
>   2 files changed, 19 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 5e3cdc9f3a9e78..c64365169b678d 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1705,6 +1705,17 @@ iommu_group_alloc_default_domain(struct iommu_group *group, int req_type)
>   
>   	lockdep_assert_held(&group->mutex);
>   
> +	/*
> +	 * Allow legacy drivers to specify the domain that will be the default
> +	 * domain. This should always be either an IDENTITY or PLATFORM domain.
> +	 * Do not use in new drivers.
> +	 */
> +	if (bus->iommu_ops->default_domain) {
> +		if (req_type)
> +			return ERR_PTR(-EINVAL);
> +		return bus->iommu_ops->default_domain;
> +	}
> +
>   	if (req_type)
>   		return __iommu_group_alloc_default_domain(bus, group, req_type);
>   
> @@ -1967,7 +1978,8 @@ void iommu_domain_free(struct iommu_domain *domain)
>   	if (domain->type == IOMMU_DOMAIN_SVA)
>   		mmdrop(domain->mm);
>   	iommu_put_dma_cookie(domain);
> -	domain->ops->free(domain);
> +	if (domain->ops->free)
> +		domain->ops->free(domain);
>   }
>   EXPORT_SYMBOL_GPL(iommu_domain_free);
>   
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index e05c93b6c37fba..87aebba474e093 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -64,6 +64,7 @@ struct iommu_domain_geometry {
>   #define __IOMMU_DOMAIN_DMA_FQ	(1U << 3)  /* DMA-API uses flush queue    */
>   
>   #define __IOMMU_DOMAIN_SVA	(1U << 4)  /* Shared process address space */
> +#define __IOMMU_DOMAIN_PLATFORM	(1U << 5)
>   
>   #define IOMMU_DOMAIN_ALLOC_FLAGS ~__IOMMU_DOMAIN_DMA_FQ
>   /*
> @@ -81,6 +82,8 @@ struct iommu_domain_geometry {
>    *				  invalidation.
>    *	IOMMU_DOMAIN_SVA	- DMA addresses are shared process addresses
>    *				  represented by mm_struct's.
> + *	IOMMU_DOMAIN_PLATFORM	- Legacy domain for drivers that do their own
> + *				  dma_api stuff. Do not use in new drivers.
>    */
>   #define IOMMU_DOMAIN_BLOCKED	(0U)
>   #define IOMMU_DOMAIN_IDENTITY	(__IOMMU_DOMAIN_PT)
> @@ -91,6 +94,7 @@ struct iommu_domain_geometry {
>   				 __IOMMU_DOMAIN_DMA_API |	\
>   				 __IOMMU_DOMAIN_DMA_FQ)
>   #define IOMMU_DOMAIN_SVA	(__IOMMU_DOMAIN_SVA)
> +#define IOMMU_DOMAIN_PLATFORM	(__IOMMU_DOMAIN_PLATFORM)
>   
>   struct iommu_domain {
>   	unsigned type;
> @@ -256,6 +260,7 @@ struct iommu_iotlb_gather {
>    * @owner: Driver module providing these ops
>    * @identity_domain: An always available, always attachable identity
>    *                   translation.
> + * @default_domain: If not NULL this will always be set as the default domain.
>    */
>   struct iommu_ops {
>   	bool (*capable)(struct device *dev, enum iommu_cap);
> @@ -290,6 +295,7 @@ struct iommu_ops {
>   	unsigned long pgsize_bitmap;
>   	struct module *owner;
>   	struct iommu_domain *identity_domain;
> +	struct iommu_domain *default_domain;

I am imaging whether we can merge above two pointers into a single one.
It is either an IDENTITY or PLATFORM domain and the core will choose it
as the default domain of a group if iommu_group_alloc_default_domain()
fails to allocate one through the iommu dev_ops.

Those iommu drivers that could result in a NULL default domain could
provide such domain and guarantee that this domain is always usable for
devices.

Probably we could give it a more meaningful name? For example,
supplemental_domain or rescue_domain?

I am not sure whether this can address the NULL-default-domain issues
of all drivers this series tries to address. So just for discussion
purpose.

Best regards,
baolu