linux-kernel - Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is encrypted

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Date:   Mon, 12 Aug 2019 14:13:24 +0200
From:   Christoph Hellwig <hch@....de>
To:     Ram Pai <linuxram@...ibm.com>
Cc:     Christoph Hellwig <hch@....de>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Thiago Jung Bauermann <bauerman@...ux.ibm.com>,
        virtualization@...ts.linux-foundation.org,
        linuxppc-devel@...ts.ozlabs.org, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Jason Wang <jasowang@...hat.com>,
        David Gibson <david@...son.dropbear.id.au>,
        Alexey Kardashevskiy <aik@...ux.ibm.com>,
        Paul Mackerras <paulus@...abs.org>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>
Subject: Re: [RFC PATCH] virtio_ring: Use DMA API if guest memory is
 encrypted

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.