Muli Ben-Yehuda wrote:
On Thu, Sep 25, 2008 at 05:45:30PM +0300, Avi Kivity wrote:
Han, Weidong wrote:
Is it possible DMA into an mmio page?
I don't see why not.
Two reasons. First it makes no sense. MMIO pages don't have RAM
backing them, they have another device's register window. So the
effect of DMA'ing into an MMIO page would be for one device to DMA
into the register window of another device, which sounds to me insane.
MMIO isn't just a register window. It may be an on-device buffer. For
instance, all packets are stored in a buffer on the ne2k that's mapped
via mmio. It would seem entirely reasonable to me to program an IDE
driver to DMA directly into the devices packet buffer.
Second, and more importantly, I've seen systems where doing the above
caused a nice, immediate, reboot. So I think that unless someone comes
with a valid scenario where we need to support it or something breaks,
we'd better err on the side of caution and not map pages that should
not be DMA targets.
Xen maps the MMIO pages into the VT-d table. The system you were using
could have just been busted. I think the burden is to prove that this
is illegal (via the architecture specification).
Regards,
Anthony Liguori
Cheers,
Muli
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