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 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?

-- 
Catalin

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