Date: Wed, 27 Apr 2016 21:17:57 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: Joerg Roedel <joro@8bytes.org>, Kevin Wolf <kwolf@redhat.com>,
        Wei Liu <wei.liu2@citrix.com>, Andy Lutomirski <luto@kernel.org>,
        qemu-block@nongnu.org, Christian Borntraeger <borntraeger@de.ibm.com>,
        Jason Wang <jasowang@redhat.com>,
        Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        qemu-devel@nongnu.org, peterx@redhat.com, linux-kernel@vger.kernel.org,
        Amit Shah <amit.shah@redhat.com>, iommu@lists.linux-foundation.org,
        Stefan Hajnoczi <stefanha@redhat.com>, kvm@vger.kernel.org,
        cornelia.huck@de.ibm.com, pbonzini@redhat.com,
        virtualization@lists.linux-foundation.org,
        Anthony PERARD <anthony.perard@citrix.com>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api
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On Wed, Apr 27, 2016 at 04:15:35PM +0100, David Woodhouse wrote:
> On Wed, 2016-04-27 at 18:05 +0300, Michael S. Tsirkin wrote:
> > 
> > I really don't get it.
> > 
> > There's exactly one device that works now and needs the work-around and
> > so that we need to support, and that is virtio. It happens to have
> > exactly the same issue on all platforms.
> 
> False. We have other devices which are currently *not* translated by
> the emulated IOMMU and which aren't going to be in the short term
> either.
> 
> We also have other devices (emulated hardware NICs) to which precisely
> the same "we don't need protection" arguments apply, and which we
> *could* expose to the guest without an IOMMU translation if we really
> wanted to. It makes as much sense as exposing virtio without an IOMMU,
> going forward.

The reasons for virtio are mostly dealing legacy.
We don't need protection is a separate issue
that I'd rather drop for now.

> > Why would we want to work hard to build platform-specific
> > solutions to a problem that can be solved in 5 lines of
> > generic code?
> 
> Because it's a dirty hack in the *wrong* place.

No one came up with a better one so far :(

> -- 
> dwmw2