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SubjectRe: dma_mapping_ops for i386
Andi Kleen wrote:
> On Wednesday 27 June 2007 16:15:17 Jeremy Fitzhardinge wrote:
>
>> Andi Kleen wrote:
>>
>>> Ok, if you can do it without ifdefs.
>>>
>>>
>> That should be OK. All the existing i386 mapping operations would just
>> have their own ops structure, right?
>>
>
> I just mention it because many people's ideas of merging files
> seem to add lots of ifdefs which is imho the totally wrong thing
> to do.
>
>
>>> And no swiotlb on i386; that is something that is completely broken
>>> in upstream Xen and needs to be fixed properly anyways.
>>>
>>>
>> Hm, OK. I'm not really familiar with the issues here. What are they?
>> Looks like Jan has made a number of Xen-ish changes to lib/swiotlb.c;
>> are more changes be needed?
>>
>
> See the recent "quiet down swiotlb warnings" thread which uncovered
> quite some corpses in Xen's current IO setup.
>
> Xen apparently bounces for multi page IOs which get merged from block
> lists because the block layer doesn't know they are not really
> continuous in machine memory.
>
> Proper fix is to tell the block layer to not merge in the first
> place instead.
>
> And probably some similar mechanism for network drivers that limits
> MTUs.
>

Well, I think there are two issues here. One is that two
pseudo-physical pages won't necessarily be contigious in bus space,
because of the pseudo-phys to machine mapping.

The second problem is that devices which can't address all machine
physical memory (ie, 32-bit PCI devices on machines with >4G memory)
will need to have bouncebuffers established for them. Device drivers
won't necessarily be able to do it because they're not really aware of
machine addresses.

> Maybe we'll still need a simple bouncing mechanism for other obscure
> devices with large IOs then, but I would very much prefer if it wasn't
> swiotlb and could be solved some other way.
>

I think 32-bit-only devices are a bigger concern, no?

J
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