Re: [PATCH 16/20] iommu/amd: Optimize map_sg and unmap_sg

[Robin Murphy]

On 08/07/16 12:45, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@xxxxxxx>
> 
> Optimize these functions so that they need only one call
> into the address alloctor. This also saves a couple of
> io-tlb flushes in the unmap_sg path.
> 
> Signed-off-by: Joerg Roedel <jroedel@xxxxxxx>
> ---
>  drivers/iommu/amd_iommu.c | 77 ++++++++++++++++++++++++++++++++---------------
>  1 file changed, 52 insertions(+), 25 deletions(-)
> 
> diff --git a/drivers/iommu/amd_iommu.c b/drivers/iommu/amd_iommu.c
> index 78b278b..e5f8e7f 100644
> --- a/drivers/iommu/amd_iommu.c
> +++ b/drivers/iommu/amd_iommu.c
> @@ -2409,45 +2409,70 @@ static void unmap_page(struct device *dev, dma_addr_t dma_addr, size_t size,
>   * lists).
>   */
>  static int map_sg(struct device *dev, struct scatterlist *sglist,
> -		  int nelems, enum dma_data_direction dir,
> +		  int nelems, enum dma_data_direction direction,
>  		  struct dma_attrs *attrs)
>  {
> +	int mapped_pages = 0, npages = 0, prot = 0, i;
> +	unsigned long start_addr, address;
>  	struct protection_domain *domain;
> -	int i;
> +	struct dma_ops_domain *dma_dom;
>  	struct scatterlist *s;
> -	phys_addr_t paddr;
> -	int mapped_elems = 0;
>  	u64 dma_mask;
>  
>  	domain = get_domain(dev);
>  	if (IS_ERR(domain))
>  		return 0;
>  
> +	dma_dom  = domain->priv;
>  	dma_mask = *dev->dma_mask;
>  
> +	for_each_sg(sglist, s, nelems, i)
> +		npages += iommu_num_pages(sg_phys(s), s->length, PAGE_SIZE);

This fails to account for the segment boundary mask[1]. Given a typical
sglist from the block layer where the boundary mask is 64K, the first
segment is 8k long, and subsequent segments are 64K long, those
subsequent segments will end up with misaligned addresses which certain
hardware may object to.

> +	address = dma_ops_alloc_iova(dev, dma_dom, npages, dma_mask);

Since a typical dma_map_sg() call is likely to involve >128K worth of
data, I wonder if it's worth going directly to a slow-path IOVA
allocation...

> +	if (address == DMA_ERROR_CODE)
> +		goto out_err;
> +
> +	start_addr = address;
> +	prot       = dir2prot(direction);
> +
>  	for_each_sg(sglist, s, nelems, i) {
> -		paddr = sg_phys(s);
> +		int j, pages = iommu_num_pages(sg_phys(s), s->length, PAGE_SIZE);
> +
> +		for (j = 0; j < pages; ++j) {
> +			unsigned long bus_addr, phys_addr;
> +			int ret;
>  
> -		s->dma_address = __map_single(dev, domain->priv,
> -					      paddr, s->length, dir, dma_mask);
> +			bus_addr  = address + (j << PAGE_SHIFT);
> +			phys_addr = (sg_phys(s) & PAGE_MASK) + (j << PAGE_SHIFT);
> +			ret = iommu_map_page(domain, bus_addr, phys_addr, PAGE_SIZE, prot, GFP_ATOMIC);
> +			if (ret)
> +				goto out_unmap;
> +
> +			mapped_pages += 1;
> +		}
>  
> -		if (s->dma_address) {
> -			s->dma_length = s->length;
> -			mapped_elems++;
> -		} else
> -			goto unmap;
> +		s->dma_address = address + s->offset;
> +		s->dma_length  = s->length;
> +		address += pages << PAGE_SHIFT;
>  	}
>  
> -	return mapped_elems;
> +	return nelems;
>  
> -unmap:
> -	for_each_sg(sglist, s, mapped_elems, i) {
> -		if (s->dma_address)
> -			__unmap_single(domain->priv, s->dma_address,
> -				       s->dma_length, dir);
> -		s->dma_address = s->dma_length = 0;
> +
> +out_unmap:
> +	pr_err("%s: IOMMU mapping error in map_sg (io-pages: %d)\n",
> +	       dev_name(dev), npages);
> +
> +	for (i = 0; i < mapped_pages; ++i) {
> +		iommu_unmap_page(domain,
> +				 start_addr + (i << PAGE_SHIFT),
> +				 PAGE_SIZE);
>  	}
>  
> +	free_iova_fast(&dma_dom->iovad, start_addr, npages);
> +
> +out_err:
>  	return 0;
>  }
>  
> @@ -2460,18 +2485,20 @@ static void unmap_sg(struct device *dev, struct scatterlist *sglist,
>  		     struct dma_attrs *attrs)
>  {
>  	struct protection_domain *domain;
> +	unsigned long startaddr;
>  	struct scatterlist *s;
> -	int i;
> +	int i,npages = 0;
>  
>  	domain = get_domain(dev);
>  	if (IS_ERR(domain))
>  		return;
>  
> -	for_each_sg(sglist, s, nelems, i) {
> -		__unmap_single(domain->priv, s->dma_address,
> -			       s->dma_length, dir);
> -		s->dma_address = s->dma_length = 0;
> -	}
> +	for_each_sg(sglist, s, nelems, i)
> +		npages += iommu_num_pages(sg_phys(s), s->length, PAGE_SIZE);

...which would also then allow this to be further simplified down to the
find_iova() trick we use in iommu-dma.

Robin.

[1]:http://article.gmane.org/gmane.linux.kernel.iommu/10553 - almost the
1-year anniversary of you making much the same comment to me :D

> +
> +	startaddr = sg_dma_address(sglist) & PAGE_MASK;
> +
> +	__unmap_single(domain->priv, startaddr, npages << PAGE_SHIFT, dir);
>  }
>  
>  /*
>