Re: [PATCH v3 1/1] iommu-api: Add map_sg/unmap_sg functions

[Will Deacon <will.deacon-5wv7dgnIgG8@public.gmane.org>]

Hi Olav,

On Mon, Jul 28, 2014 at 07:38:51PM +0100, Olav Haugan wrote:
> Mapping and unmapping are more often than not in the critical path.
> map_sg and unmap_sg allows IOMMU driver implementations to optimize
> the process of mapping and unmapping buffers into the IOMMU page tables.
> 
> Instead of mapping a buffer one page at a time and requiring potentially
> expensive TLB operations for each page, this function allows the driver
> to map all pages in one go and defer TLB maintenance until after all
> pages have been mapped.
> 
> Additionally, the mapping operation would be faster in general since
> clients does not have to keep calling map API over and over again for
> each physically contiguous chunk of memory that needs to be mapped to a
> virtually contiguous region.
> 
> Signed-off-by: Olav Haugan <ohaugan-sgV2jX0FEOL9JmXXK+q4OQ@public.gmane.org>
> ---
>  drivers/iommu/iommu.c | 48 ++++++++++++++++++++++++++++++++++++++++++++++++
>  include/linux/iommu.h | 28 ++++++++++++++++++++++++++++
>  2 files changed, 76 insertions(+)
> 
> diff --git a/drivers/iommu/iommu.c b/drivers/iommu/iommu.c
> index 1698360..cd65511 100644
> --- a/drivers/iommu/iommu.c
> +++ b/drivers/iommu/iommu.c
> @@ -1088,6 +1088,54 @@ size_t iommu_unmap(struct iommu_domain *domain, unsigned long iova, size_t size)
>  }
>  EXPORT_SYMBOL_GPL(iommu_unmap);
>  
> +int iommu_map_sg(struct iommu_domain *domain, unsigned long iova,
> +			struct scatterlist *sg, unsigned int nents,
> +			int prot, unsigned long flags)
> +{
> +	int ret = 0;
> +	unsigned long offset = 0;
> +
> +	BUG_ON(iova & (~PAGE_MASK));
> +
> +	if (unlikely(domain->ops->map_sg == NULL)) {
> +		unsigned int i;
> +		struct scatterlist *s;
> +
> +		for_each_sg(sg, s, nents, i) {
> +			phys_addr_t phys = page_to_phys(sg_page(s));
> +			u32 page_len = PAGE_ALIGN(s->offset + s->length);

Hmm, this is a pretty horrible place where CPU page size (from the sg list)
meets the IOMMU and I think we need to do something better to avoid spurious
failures. In other words, the sg list should be iterated in such a way that
we always pass a multiple of a supported iommu page size to iommu_map.

All the code using PAGE_MASK and PAGE_ALIGN needn't match what is supported
by the IOMMU hardware.

Will