From mboxrd@z Thu Jan  1 00:00:00 1970
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Subject: Re: [RFC 7/11] virtio_pci: new, capability-aware driver.
Date: Wed, 11 Jan 2012 20:10:19 +1100
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: Pawel Moll <pawel.moll@arm.com>, "Michael S. Tsirkin" <mst@redhat.com>, virtualization <virtualization@lists.linux-foundation.org>, Christian Borntraeger <borntraeger@de.ibm.com>, Sasha Levin <levinsasha928@gmail.com>
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On Wed, 2012-01-11 at 08:47 +0000, Stefan Hajnoczi wrote:
> 
> This is also an opportunity to stop using CPU physical addresses in
> the ring and instead perform DMA like a normal PCI device (use bus
> addresses).

Euh why ?

That would mean in many cases adding a layer of iommu, which will slow
things down a lot ... unless we create a special virtio bus which has
its own dma-ops that do a direct v:p translation that is. But anything
PCI doing dma_map_sg and co will involved emulated iommu mapping &
unmapping and will be a huge hit on performances.

Cheers,
Ben.