Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices

From: Christoph Hellwig <hch@infradead.org>
To: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Ram Pai <linuxram@us.ibm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Christoph Hellwig <hch@infradead.org>,
	robh@kernel.org, pawel.moll@arm.com,
	Tom Lendacky <thomas.lendacky@amd.com>,
	aik@ozlabs.ru, jasowang@redhat.com, cohuck@redhat.com,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, joe@perches.com,
	"Rustad, Mark D" <mark.d.rustad@intel.com>,
	david@gibson.dropbear.id.au, linuxppc-dev@lists.ozlabs.org,
	elfring@users.sourceforge.net,
	Anshuman Khandual <khandual@linux.vnet.ibm.com>
Subject: Re: [RFC V2] virtio: Add platform specific DMA API translation for virito devices
Date: Wed, 13 Jun 2018 00:41:41 -0700	[thread overview]
Message-ID: <20180613074141.GA12033@infradead.org> (raw)
In-Reply-To: <07b804fccd7373c650be79ac9fa77ae7f2375ced.camel@kernel.crashing.org>

On Mon, Jun 11, 2018 at 01:29:18PM +1000, Benjamin Herrenschmidt wrote:
> At the risk of repeating myself, let's just do the first pass which is
> to switch virtio over to always using the DMA API in the actual data
> flow code, with a hook at initialization time that replaces the DMA ops
> with some home cooked "direct" ops in the case where the IOMMU flag
> isn't set.
> 
> This will be equivalent to what we have today but avoids having 2
> separate code path all over the driver.
> 
> Then a second stage, I think, is to replace this "hook" so that the
> architecture gets a say in the matter.

I don't think we can actually use dma_direct_ops.  It still allows
architectures to override parts of the dma setup, which virtio seems
to blindly assume phys == dma and not cache flushing.

I think the right way forward is to either add a new
VIRTIO_F_IS_PCI_DEVICE (or redefine the existing iommu flag if deemed
possible).  And then make sure recent qemu always sets it.