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

[Anthony Liguori <anthony@codemonkey.ws>]

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.
>>
>> 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."

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.

Regards,

Anthony Liguori

> However I don't think guests use this with devices we emulate in any way.
>
> Haven't looked at ISA.
>