On Fri, Nov 09, 2018 at 02:41:18PM +0000, Robin Murphy wrote: >> - >> #define CMD_SET_TYPE(cmd, t) ((cmd)->data[1] |= ((t) << 28)) >> #define LOOP_TIMEOUT 100000 >> @@ -2339,7 +2337,7 @@ static dma_addr_t __map_single(struct device *dev, >> paddr &= PAGE_MASK; >> address = dma_ops_alloc_iova(dev, dma_dom, pages, dma_mask); >> - if (address == AMD_IOMMU_MAPPING_ERROR) >> + if (address == DMA_MAPPING_ERROR) > > This for one is clearly broken, because the IOVA allocator still returns 0 > on failure here... Indeed. And that shows how the original code was making a mess of these different constants.. > I very much agree with the concept, but I think the way to go about it is > to convert the implementations which need it to the standardised > *_MAPPING_ERROR value one-by-one, and only then then do the big sweep to > remove them all. That has more of a chance of getting worthwhile review and > testing from the respective relevant parties (I'll confess I came looking > for this bug specifically, since I happened to recall amd_iommu having a > tricky implicit reliance on the old DMA_ERROR_CODE being 0 on x86). I'll see if I can split this out somehow, but I'm not sure it is going to be all that much more readable.. > In terms of really minimising the error-checking overhead it's a bit of a > shame that DMA_MAPPING_ERROR = 0 doesn't seem viable as the thing to > standardise on, since that has advantages at the micro-optimisation level > for many ISAs - fixing up the legacy IOMMU code doesn't seem > insurmountable, but I suspect there may well be non-IOMMU platforms where > DMA to physical address 0 is a thing :( Yes, that is what I'm more worried about. > (yeah, I know saving a couple of instructions and potential register > allocations is down in the noise when we're already going from an indirect > call to an inline comparison; I'm mostly just thinking out loud there) The nice bit of standardizing the value is that we get rid of an indirect call, which generally is much more of a problem at the micro-architecture level.
