linux-kernel - Re: [PATCH v2 10/11] xen/arm: introduce phys/dma translations in xen_dma_sync_for

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Date:   Mon, 8 Jun 2020 22:38:02 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Christoph Hellwig <hch@...radead.org>, jgross@...e.com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        tamas@...engyel.com, roman@...eda.com,
        Stefano Stabellini <stefano.stabellini@...inx.com>
Subject: Re: [PATCH v2 10/11] xen/arm: introduce phys/dma translations in
 xen_dma_sync_for_*

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.