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SubjectRe: [00/15] swiotlb cleanup

* 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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