Messages in this thread | | | Date | Wed, 10 Sep 2008 22:03:32 +0900 | Subject | Re: [PATCH] x86: avoid unnecessary low zone allocation in AMD IOMMU's alloc_coherent | From | FUJITA Tomonori <> |
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On Wed, 10 Sep 2008 14:48:22 +0200 Joerg Roedel <joerg.roedel@amd.com> wrote:
> On Wed, Sep 10, 2008 at 09:38:11PM +0900, FUJITA Tomonori wrote: > > On Wed, 10 Sep 2008 14:03:10 +0200 > > Joerg Roedel <joerg.roedel@amd.com> wrote: > > > It needs a fix anyway and the > > > right solution here is to fall back to one of the software iommu > > > implementations. The stackable dma_ops patches I have currently in work > > > will do exactly that. > > > > I'm not sure you need the stackable dma_ops support. Calgary IOMMU had > > the same problem and already solved it with dma_ops-per-device option. > > We need stackable dma_ops anyway for paravirt IOMMU support in KVM.
I know. We discussed it when adding dma_ops-per-device support.
> And they will fix this issue too.
Ok, I'll wait until I see how the patches solve the problem cleanly.
> > > These flags are already removed in the dma_alloc_coherent function which > > > calls this one. Further I think in the case of a remapping IOMMU like > > > > Not true about x86/tip/iommu. dma_alloc_coherent in dma-mapping.h does > > that so that swiotlb and pci-nommu don't need the gfp hack. Clearing > > the gfp flags is much simpler than setting up the flags correctly > > mainly because of the fallback device, setting up the flags is really > > difficult. > > Yes, dma_alloc_coherent in dma-mapping.h clears the flags. And this > function also calls ops->alloc_coherent which points to the AMD IOMMUs > alloc_coherent function if the driver is in place.
Hmm, I'm not sure what code you look at. Here's dma_alloc_coherent() in tip/x86/iommu:
dma_alloc_coherent(struct device *dev, size_t size, dma_addr_t *dma_handle, gfp_t gfp) { struct dma_mapping_ops *ops = get_dma_ops(dev); void *memory;
gfp &= ~(__GFP_DMA | __GFP_HIGHMEM | __GFP_DMA32);
Surely we here clear the flag but...
if (dma_alloc_from_coherent(dev, size, dma_handle, &memory)) return memory;
if (!dev) { dev = &x86_dma_fallback_dev; gfp |= GFP_DMA; }
we play with it here though (not happens with pci devices),
if (!dev->dma_mask) return NULL;
if (!ops->alloc_coherent) return NULL;
Then dma_alloc_coherent_gfp_flags() sets it again according to device->coherent_dma_mask and gfp before ops->alloc_coherent hook:
return ops->alloc_coherent(dev, size, dma_handle, dma_alloc_coherent_gfp_flags(dev, gfp));
This code can set up the exact same gfp flag for swiotbl and nommu as before.
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