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Re: [Qemu-devel] [PATCH RFC] vga: flag vga ram for notifiers

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH RFC] vga: flag vga ram for notifiers
Date: Thu, 31 Mar 2011 19:44:35 -0500
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On 03/31/2011 06:42 PM, Michael S. Tsirkin wrote:
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.


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."

What's not clear to me though, is whether peer-to-peer transactions are done via a special PCI mechanism or whether it's done by doing a I/O access to the address that the device happens to be mapped to.


Anthony Liguori

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

Haven't looked at ISA.

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