linux-kernel - Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

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Date:	Wed, 27 Apr 2016 21:17:57 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	David Woodhouse <dwmw2@...radead.org>
Cc:	Joerg Roedel <joro@...tes.org>, Kevin Wolf <kwolf@...hat.com>,
	Wei Liu <wei.liu2@...rix.com>,
	Andy Lutomirski <luto@...nel.org>, qemu-block@...gnu.org,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Jason Wang <jasowang@...hat.com>,
	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	qemu-devel@...gnu.org, peterx@...hat.com,
	linux-kernel@...r.kernel.org, Amit Shah <amit.shah@...hat.com>,
	iommu@...ts.linux-foundation.org,
	Stefan Hajnoczi <stefanha@...hat.com>, kvm@...r.kernel.org,
	cornelia.huck@...ibm.com, pbonzini@...hat.com,
	virtualization@...ts.linux-foundation.org,
	Anthony PERARD <anthony.perard@...rix.com>
Subject: Re: [PATCH V2 RFC] fixup! virtio: convert to use DMA api

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