From mboxrd@z Thu Jan  1 00:00:00 1970
From: Christoph Hellwig <hch@lst.de>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted
Date: Mon, 12 Aug 2019 14:13:24 +0200
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To: Ram Pai <linuxram@us.ibm.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Alexey Kardashevskiy <aik@linux.ibm.com>, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, Paul Mackerras <paulus@ozlabs.org>, iommu@lists.linux-foundation.org, linuxppc-devel@lists.ozlabs.org, Christoph Hellwig <hch@lst.de>, David Gibson <david@gibson.dropbear.id.au>
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On Sat, Aug 10, 2019 at 11:46:21PM -0700, Ram Pai wrote:
> If the hypervisor (hardware for hw virtio devices) does not mandate a
> DMA API, why is it illegal for the driver to request, special handling
> of its i/o buffers? Why are we associating this special handling to
> always mean, some DMA address translation? Can't there be 
> any other kind of special handling needs, that has nothing to do with
> DMA address translation?

I don't think it is illegal per se.  It is however completely broken
if we do that decision on a system weide scale rather than properly
requesting it through a per-device flag in the normal virtio framework.