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

[Catalin Marinas <catalin.marinas@xxxxxxx>]

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()?

> 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.

-- 
Catalin

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