Re: [PATCH 07/20] iommu/amd: Remove special mapping code for dma_ops path

From: Joerg Roedel
Date: Tue Jul 12 2016 - 07:08:45 EST

Hi Robin,

On Tue, Jul 12, 2016 at 11:55:39AM +0100, Robin Murphy wrote:
> > start = address;
> > for (i = 0; i < pages; ++i) {
> > - ret = dma_ops_domain_map(dma_dom, start, paddr, dir);
> > - if (ret == DMA_ERROR_CODE)
> > + ret = iommu_map_page(&dma_dom->domain, start, paddr,
> > + PAGE_SIZE, prot, GFP_ATOMIC);
> I see that amd_iommu_map/unmap() takes a lock around calling
> iommu_map/unmap_page(), but we don't appear to do that here. That seems
> to suggest that either one is unsafe or the other is unnecessary.

At this point no locking is required, because in this code path we know
that we own the memory range and that nobody else is mapping that range.

In the IOMMU-API path we can't make that assumption, so locking is
required there. Both code-path use different types of domains, so there
is also no chance that a domain is used in both code-paths (except when
a dma-ops domain is set up).