Messages in this thread | | | Date | Fri, 10 Jul 2009 16:12:48 +0200 | From | Ingo Molnar <> | Subject | Re: [00/15] swiotlb cleanup |
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* Ian Campbell <Ian.Campbell@eu.citrix.com> wrote:
> I've not examined the series in detail it looks OK but I don't > think it is quite sufficient. The Xen determination of whether a > buffer is dma_capable or not is based on the physical address > while dma_capable takes only the dma address. > > I'm not sure we can "invert" our conditions to work back from dma > address to physical since given a start dma address and a length > we would need to check that dma_to_phys(dma+PAGE_SIZE) == > dma_to_phys(dma)+PAGE_SIZE etc. However dma+PAGE_SIZE might belong > to a different domain so translating it to a physical address in > isolation tells us nothing especially useful since it would give > us the physical address in that other guest which is useless to > us. If we could pass both physical and dma address to dma_capable > I think that would probably be sufficient for our purposes. > > As well as that Xen needs some way to influence the allocation of > the actual bounce buffer itself since we need to arrange for it to > be machine address contiguous as well as physical address > contiguous. This series explicitly removes those hooks without > replacement. My most recent proposal was to have a new > swiotlb_init variant which was given a preallocated buffer which > this series doesn't necessarily preclude. > > The phys_to_dma and dma_to_phys translation points are the last > piece Xen needs and seem to be preserved in this series. > > However Fujita's objection to all of the previous swiotlb-for-xen > proposals was around the addition of the Xen hooks in whichever > location. Originally these hooks were via __weak functions and > later proposals implemented them via function pointer hooks in the > x86 implementations of the arch-abstract interfaces (phys<->dma > and dma_capable etc). I don't think this series addresses those > objections (fair enough -- it wasn't intended to) or leads to any > new approach to solving the issue, although I also don't think it > makes the issue any harder to address. I don't think it will be > possible to make progress on Xen usage of swiotlb until a solution > can be found to this conflict of opinion. > > Fujita suggested that we export the core sync_single() > functionality and reimplemented the surrounding infrastructure in > terms of that (and incorporating our additional requirements). I > prototyped this (it is currently unworking, in fact it seems to > have developed rather a taste for filesystems :-() but the > diffstat of my WIP patch is: > > arch/x86/kernel/pci-swiotlb.c | 6 > arch/x86/xen/pci-swiotlb.c | 2 > drivers/pci/xen-iommu.c | 385 ++++++++++++++++++++++++++++++++++++++++-- > include/linux/swiotlb.h | 12 + > lib/swiotlb.c | 10 - > 5 files changed, 385 insertions(+), 30 deletions(-) > > where a fair number of the lines in xen-iommu.c are copies of > functions from swiotlb.c with minor modifications. As I say it > doesn't work yet but I think it's roughly indicative of what such > an approach would look like. I don't like it much but am happy to > run with it if it looks to be the most acceptable approach. [...]
+400 lines of code to avoid much fewer lines of generic code impact on the lib/swiotlb.c side sounds like a bad technical choice to me.
It makes the swiotlb code less useful and basically forks a random implementation of it in drivers/pci/xen-iommu.c.
Fujita-san, can you think of a solution that avoids the whole-sale copying of hundreds of lines of code?
Ingo
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