Re: [PATCH v2 10/11] xen/arm: introduce phys/dma translations in xen_dma_sync_for_*

[Christoph Hellwig <hch@xxxxxxxxxxxxx>]

On Mon, Jun 08, 2020 at 05:38:28PM -0700, Stefano Stabellini wrote:
> Yeah, the pfn_valid check is a bit weird by definition because we are
> using it to understand whether the address belong to us or to another
> VM. To do the pfn_valid check we need to translate the dma address into
> something the CPU understands, hence, the dma_to_phys call.
> 
> Why can't we use the already-provided paddr? Because paddr has been
> translated twice:
> 1) from dma to maybe-foreign phys address (could be ours, or another VM)
> 2) from maybe-foreign address to local (using our local mapping of the 
> foreign page)
> 
> In fact, it would be clearer if we had all three addresses as parameters
> of xen_dma_sync_for_cpu: the dma address, the maybe-foreign physical
> address (we tend to call it xenbus address, baddr), the local physical
> address. Something like:

I think instead we should move the arch_sync_dma_for_{device,cpu}
calls from xen_dma_sync_for_{device,cpu} into the callers, as they
are provided by the generic dma-noncoherent.h and optimized out for
coherent architectures like x86.  Then the swiotlb-xen.c code only
need to call dma_cache_maint as the interface (which would have to
grow a better name), which should then only need a single kind of
address.