[RFC] add a struct page* parameter to dma_map_ops.unmap_page

[Mitchel Humpherys]

On Fri, Nov 21 2014 at 03:48:33 AM, Stefano Stabellini <stefano.stabellini at eu.citrix.com> wrote:
> On Mon, 17 Nov 2014, Stefano Stabellini wrote:
>> Hi all,
>> I am writing this email to ask for your advice.
>> 
>> On architectures where dma addresses are different from physical
>> addresses, it can be difficult to retrieve the physical address of a
>> page from its dma address.
>> 
>> Specifically this is the case for Xen on arm and arm64 but I think that
>> other architectures might have the same issue.
>> 
>> Knowing the physical address is necessary to be able to issue any
>> required cache maintenance operations when unmap_page,
>> sync_single_for_cpu and sync_single_for_device are called.
>> 
>> Adding a struct page* parameter to unmap_page, sync_single_for_cpu and
>> sync_single_for_device would make Linux dma handling on Xen on arm and
>> arm64 much easier and quicker.
>> 
>> I think that other drivers have similar problems, such as the Intel
>> IOMMU driver having to call find_iova and walking down an rbtree to get
>> the physical address in its implementation of unmap_page.
>> 
>> Callers have the struct page* in their hands already from the previous
>> map_page call so it shouldn't be an issue for them.  A problem does
>> exist however: there are about 280 callers of dma_unmap_page and
>> pci_unmap_page. We have even more callers of the dma_sync_single_for_*
>> functions.
>> 
>> 
>> 
>> Is such a change even conceivable? How would one go about it?
>> 
>> I think that Xen would not be the only one to gain from it, but I would
>> like to have a confirmation from others: given the magnitude of the
>> changes involved I would actually prefer to avoid them unless multiple
>> drivers/archs/subsystems could really benefit from them.
>
> Given the lack of interest from the community, I am going to drop this
> idea.

Actually it sounds like the right API design to me.  As a bonus it
should help performance a bit as well.  For example, the current
implementations of dma_sync_single_for_{cpu,device} and dma_unmap_page
on ARM while using the IOMMU mapper
(arm_iommu_sync_single_for_{cpu,device}, arm_iommu_unmap_page) all call
iommu_iova_to_phys which generally results in a page table walk or a
hardware register write/poll/read.

The problem, as you mentioned, is that there are a ton of callers of the
existing APIs.  I think David Vrabel had a good suggestion for dealing
with this:

On Mon, Nov 17 2014 at 06:43:46 AM, David Vrabel <david.vrabel at citrix.com> wrote:
> You may need to consider a parallel set of map/unmap API calls that
> return/accept a handle, and then converting drivers one-by-one as
> required, instead of trying to convert every single driver at once.

However, I'm not sure whether the costs of having a parallel set of APIs
outweigh the benefits of a cleaner API and a slight performance boost...
But I hope the idea isn't completely abandoned without some profiling or
other evidence of its benefits (e.g. patches showing how drivers could
be simplified with the new APIs).


-Mitch

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