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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

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 -> 
????



> Currently arm64_swiotlb_dma_ops assume cache-coherent DMA. I have a
> patch which introduces new ops for non-coherent DMA but this should
> really be orthogonal to swiotlb. You can basically have 4 combinations
> of coherent/non-coherent and swiotlb/iommu. Mark Rutland is currently
> looking into how best to describe this via DT as it may not even be per
> SoC but per bus or device.

It seems to me that calling arm64_swiotlb_dma_ops.alloc would ensure
that we allocate the right buffer for the right device?

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