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

From: Christoph Hellwig <hch@infradead.org>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: Christoph Hellwig <hch@infradead.org>,
	jgross@suse.com, boris.ostrovsky@oracle.com,
	konrad.wilk@oracle.com, xen-devel@lists.xenproject.org,
	linux-kernel@vger.kernel.org, tamas@tklengyel.com,
	roman@zededa.com,
	Stefano Stabellini <stefano.stabellini@xilinx.com>
Subject: Re: [PATCH v2 10/11] xen/arm: introduce phys/dma translations in xen_dma_sync_for_*
Date: Mon, 8 Jun 2020 22:39:49 -0700	[thread overview]
Message-ID: <20200609053949.GA26473@infradead.org> (raw)
In-Reply-To: <20200609053802.GB3015@infradead.org>

On Mon, Jun 08, 2020 at 10:38:02PM -0700, Christoph Hellwig wrote:
> 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.

... actually I'd keep the xen_dma_sync_for_{device,cpu} names for the
low-level interface, just move the arch_sync_dma_for_{device,cpu}
calls up.