Re: [PATCH 03/19] dma-iommu: don't use a scatterlist in iommu_dma_alloc

[Christoph Hellwig]

On Wed, Feb 06, 2019 at 03:28:28PM +0000, Robin Murphy wrote:
> Because if iommu_map() only gets called at PAGE_SIZE granularity, then the 
> IOMMU PTEs will be created at PAGE_SIZE (or smaller) granularity, so any 
> effort to get higher-order allocations matching larger IOMMU block sizes is 
> wasted, and we may as well have just done this:
>
> 	for (i = 0; i < count; i++) {
> 		struct page *page = alloc_page(gfp);
> 		...
> 		iommu_map(..., page_to_phys(page), PAGE_SIZE, ...);
> 	}

True.  I've dropped this patch.

> Really, it's a shame we have to split huge pages for the CPU remap, since 
> in the common case the CPU MMU will have a matching block size, but IIRC 
> there was something in vmap() or thereabouts that explicitly chokes on 
> them.

That just needs a volunteer to fix the implementation, as there is no
fundamental reason not to remap large pages.