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

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Date:   Mon, 11 Feb 2019 17:00:49 +0100
From:   Christoph Hellwig <hch@....de>
To:     Robin Murphy <robin.murphy@....com>
Cc:     Christoph Hellwig <hch@....de>, Joerg Roedel <joro@...tes.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Tom Lendacky <thomas.lendacky@....com>,
        iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/19] dma-iommu: don't use a scatterlist in
 iommu_dma_alloc

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.