Re: [RFC PATCH] dma-iommu: Add iommu_dma_max_mapping_size()

[Christoph Hellwig]

On Mon, May 16, 2022 at 09:06:01PM +0800, John Garry wrote:
> For streaming DMA mappings involving an IOMMU and whose IOVA len regularly
> exceeds the IOVA rcache upper limit (meaning that they are not cached),
> performance can be reduced.
> 
> Add the IOMMU callback for DMA mapping API dma_max_mapping_size(), which
> allows the drivers to know the mapping limit and thus limit the requested 
> IOVA lengths.
> 
> This resolves the performance issue originally reported in [0] for a SCSI
> HBA driver which was regularly mapping SGLs which required IOVAs in
> excess of the IOVA caching limit. In this case the block layer limits the
> max sectors per request - as configured in __scsi_init_queue() - which
> will limit the total SGL length the driver tries to map and in turn limits
> IOVA lengths requested.
> 
> [0] https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@xxxxxxxxxx/
> 
> Signed-off-by: John Garry <john.garry@xxxxxxxxxx>
> ---
> Sending as an RFC as iommu_dma_max_mapping_size() is a soft limit, and not
> a hard limit which I expect is the semantics of dma_map_ops.max_mapping_size
> 
> diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c
> index 09f6e1c0f9c0..e2d5205cde37 100644
> --- a/drivers/iommu/dma-iommu.c
> +++ b/drivers/iommu/dma-iommu.c
> @@ -1442,6 +1442,21 @@ static unsigned long iommu_dma_get_merge_boundary(struct device *dev)
>  	return (1UL << __ffs(domain->pgsize_bitmap)) - 1;
>  }
>  

> +	if (!domain)
> +		return 0;
> +
> +	cookie = domain->iova_cookie;
> +	if (!cookie || cookie->type != IOMMU_DMA_IOVA_COOKIE)
> +		return 0;

Can these conditions even be true here?

> +static inline unsigned long iova_rcache_range(void)
> +{
> +	return 0;
> +}

Given that IOMMU_DMA select IOMMU_IOVA there is no need for this stub.

Otherwise this looks sensible to me.