Re: [Qemu-devel] [PATCH RFC v2 2/2] hw/pci: handle unassigned pci addresses

[Peter Maydell]

On 9 September 2013 18:09, Jan Kiszka <address@hidden> wrote:
> On 2013-09-09 18:58, Peter Maydell wrote:
>> Why is a DMA request any different from any other communication
>> between two devices?
>
> Other communication between devices requiring to take the target
> device's lock while holding the one of the initiator will be a no-go as
> well. But usually these scenarios are clearly defined, not
> guest-influenceable and can be avoided by the initiator.

How? If I'm a device and I need to raise a GPIO output line
I have no idea what the other end is connected to. Similarly
for more interesting device-to-device connections than
pure on-or-off signal lines.

> DMA is too
> generic. E.g., the guest can easily program a device to
> "accidentally" hit another device's MMIO region

This is just a special case of generic device-device interaction.

-- PMM