Re: [Xen-devel] [PATCH v5 11/13] xen: introduce xen_alloc/free_coherent_pages

[Catalin Marinas <catalin.marinas@xxxxxxx>]

On 6 Sep 2013, at 17:52, Stefano Stabellini <Stefano.Stabellini@xxxxxxxxxxxxx> 
wrote:
> On Fri, 6 Sep 2013, Catalin Marinas wrote:
>> On Fri, Sep 06, 2013 at 05:09:52PM +0100, Stefano Stabellini wrote:
>>> On Fri, 6 Sep 2013, Catalin Marinas wrote:
>>>> On Fri, Sep 06, 2013 at 03:59:02PM +0100, Stefano Stabellini wrote:
>>>>> On Fri, 6 Sep 2013, Catalin Marinas wrote:
>>>>>> On Thu, Sep 05, 2013 at 05:43:33PM +0100, Stefano Stabellini wrote:
>>>>>>> On Thu, 5 Sep 2013, Catalin Marinas wrote:
>>>>>>>> On Thu, Aug 29, 2013 at 07:32:32PM +0100, Stefano Stabellini wrote:
>>>>>>>>> xen_swiotlb_alloc_coherent needs to allocate a coherent buffer for cpu
>>>>>>>>> and devices. On native x86 and ARMv8 is sufficient to call
>>>>>>>>> __get_free_pages in order to get a coherent buffer, while on ARM we 
>>>>>>>>> need
>>>>>>>>> to call arm_dma_ops.alloc.
>>>>>>>> 
>>>>>>>> Don't bet on this for ARMv8. It's not mandated for the architecture, so
>>>>>>>> at some point some SoC will require non-cacheable buffers for 
>>>>>>>> coherency.
>>>>>>> 
>>>>>>> I see.
>>>>>>> Would it be better if I implemented xen_alloc_coherent_pages on armv8 by
>>>>>>> calling arm64_swiotlb_dma_ops.alloc?
>>>>>> 
>>>>>> What does this buffer do exactly? Is it allocated by guests?
>>>>> 
>>>>> It is allocated by Dom0 to do DMA to/from a device.
>>>>> It is the buffer that is going to be returned by dma_map_ops.alloc to
>>>>> the caller:
>>>>> 
>>>>> On x86:
>>>>> dma_map_ops.alloc -> xen_swiotlb_alloc_coherent -> 
>>>>> xen_alloc_coherent_pages -> __get_free_pages
>>>>> 
>>>>> On ARM:
>>>>> dma_map_ops.alloc -> xen_swiotlb_alloc_coherent -> 
>>>>> xen_alloc_coherent_pages -> arm_dma_ops.alloc
>>>>> 
>>>>> On ARM64
>>>>> dma_map_ops.alloc -> xen_swiotlb_alloc_coherent -> 
>>>>> xen_alloc_coherent_pages -> ????
>>>> 
>>>> OK, I'm getting more confused. Do all the above calls happen in the
>>>> guest, Dom0, or a mix?
>>> 
>>> I guess the confusion comes from a difference in terminology: dom0 is a
>>> guest like the others, just a bit more privileged. We usually call domU
>>> a normal unprivileged guest.
>> 
>> Thanks for the explanation.
>> 
>>> The above calls would happen in Dom0 (when an SMMU is not available).
>> 
>> So for Dom0, are there cases when it needs xen_swiotlb_alloc_coherent()
>> and other cases when it needs the arm_dma_ops.alloc? In Dom0 could we
>> not always use the default dma_alloc_coherent()?
> 
> Keep in mind that dom0 runs with second stage translation enabled.  This
> means that what dom0 thinks is a physical address (machine address in
> Xen terminology), it's actually just an intermediate physical address.
> Also for the same reason what dom0 thinks is a contiguous buffer, it's
> actually only contiguous in the intermediate physical address space.

OK, it makes sense now.  I thought dom0 is like the KVM host where stage
2 is disabled (or just flat).

> BTW if the Matrix is your kind of fun, I wrote an blog post explaining the
> swiotlb Morpheus style:
> http://blog.xen.org/index.php/2013/08/14/swiotlb-by-morpheus/

That was easier to understand ;)

>>> They could also happen in a DomU if we assign a physical device to it
>>> (and an SMMU is not available).
>> 
>> The problem is that you don't necessarily know one kind of coherency you
>> know for a physical device. As I said, we plan to do this DT-driven.
> 
> OK, but if I call arm64_swiotlb_dma_ops.alloc passing the right
> arguments to it, I should be able to get the right coherency for the
> right device, correct?

I think it needs a bit more work on the Xen part.  Basically
dma_alloc_attrs() calls get_dma_ops() to obtain the best DMA operations
for a device.  arm64_swiotlb_dma_ops is just the default implementation
and I'll add a _noncoherent variant as well.  Default dma_ops will be
set to one of these during boot.  But a device is also allowed to have
its own dev->archdata.dma_ops, set via set_dma_ops().

So even if you set the default dma_ops to Xen ops, you may not get them
via dma_alloc_coherent().  I don't see any easier solution other than
patching the dma_alloc_attrs() function to issue a Hyp call after the
memory has been allocated with the get_dma_ops()->alloc().  But I don't
like this either.

Catalin
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