Re: [PATCH v2 3/4] iommu: Redefine IOMMU_CAP_CACHE_COHERENCY as the cap flag for IOMMU_CACHE

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

On 2022/4/7 23:23, Jason Gunthorpe wrote:
> While the comment was correct that this flag was intended to convey the
> block no-snoop support in the IOMMU, it has become widely implemented and
> used to mean the IOMMU supports IOMMU_CACHE as a map flag. Only the Intel
> driver was different.
> 
> Now that the Intel driver is using enforce_cache_coherency() update the
> comment to make it clear that IOMMU_CAP_CACHE_COHERENCY is only about
> IOMMU_CACHE.  Fix the Intel driver to return true since IOMMU_CACHE always
> works.
> 
> The two places that test this flag, usnic and vdpa, are both assigning
> userspace pages to a driver controlled iommu_domain and require
> IOMMU_CACHE behavior as they offer no way for userspace to synchronize
> caches.
> 
> Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>
> ---
>   drivers/iommu/intel/iommu.c | 2 +-
>   include/linux/iommu.h       | 3 +--
>   2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
> index 8f3674e997df06..14ba185175e9ec 100644
> --- a/drivers/iommu/intel/iommu.c
> +++ b/drivers/iommu/intel/iommu.c
> @@ -4556,7 +4556,7 @@ static bool intel_iommu_enforce_cache_coherency(struct iommu_domain *domain)
>   static bool intel_iommu_capable(enum iommu_cap cap)
>   {
>   	if (cap == IOMMU_CAP_CACHE_COHERENCY)
> -		return domain_update_iommu_snooping(NULL);
> +		return true;
>   	if (cap == IOMMU_CAP_INTR_REMAP)
>   		return irq_remapping_enabled == 1;
>   
> diff --git a/include/linux/iommu.h b/include/linux/iommu.h
> index fe4f24c469c373..fd58f7adc52796 100644
> --- a/include/linux/iommu.h
> +++ b/include/linux/iommu.h
> @@ -103,8 +103,7 @@ static inline bool iommu_is_dma_domain(struct iommu_domain *domain)
>   }
>   
>   enum iommu_cap {
> -	IOMMU_CAP_CACHE_COHERENCY,	/* IOMMU can enforce cache coherent DMA
> -					   transactions */
> +	IOMMU_CAP_CACHE_COHERENCY,	/* IOMMU_CACHE is supported */
>   	IOMMU_CAP_INTR_REMAP,		/* IOMMU supports interrupt isolation */
>   	IOMMU_CAP_NOEXEC,		/* IOMMU_NOEXEC flag */
>   };

Reviewed-by: Lu Baolu <baolu.lu@linux.intel.com>

Best regards,
baolu