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Date: Fri, 1 Apr 2011 01:42:40 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [Qemu-devel] [PATCH RFC] vga: flag vga ram for notifiers
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To: Anthony Liguori <anthony@codemonkey.ws>
Cc: Peter Maydell <peter.maydell@linaro.org>, Alex Williamson <alex.williamson@redhat.com>, qemu-devel@nongnu.org

On Thu, Mar 31, 2011 at 04:49:46PM -0500, Anthony Liguori wrote:
> On 03/31/2011 04:38 PM, Michael S. Tsirkin wrote:
> >
> >>That seems like a clearer API, yes. I think it makes it much more
> >>obvious what it's trying to achieve.
> >>
> >>-- PMM
> >Maybe register_dma_area - its' not 100% virtio specific.
> 
> It's never been clear to me whether that's true or not.  I've heard
> mixed things about whether devices DMA to other devices.  I've never
> been able to find something in a specification stating
> authoritatively one way or another.
> 
> Regards,
> 
> Anthony Liguori
> 

AFAIK the capability of cross-talk between PCI devices
exists in PCI and is optional in PCI Express.

PCI spec says:
	Full multi-master capability allowing any PCI master peer-to-peer
	access to any PCI master/target.

The Express spec says:
	"The capability to route peer-to-peer transactions between hierarchy
	domains through a Root Complex is optional and implementation dependent.
	For example, an implementation may incorporate a real or virtual Switch
	internally within the Root Complex to enable full peer-to- peer support
	in a software transparent way."

However I don't think guests use this with devices we emulate in any way.

Haven't looked at ISA.

-- 
MST