On Thu, Sep 25, 2008 at 04:51:24PM -0500, Anthony Liguori wrote: > 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.
Unlikely, but ok. > 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. It would be insane to me. Have you tried this on real hardware and seen it work? >> 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). I strongly disagree. You are advocating something that is potentially unsafe---for the sake of code simplicity?! I am advocating caution in what we let an *untrusted* guest do. Cheers, Muli -- The First Workshop on I/O Virtualization (WIOV '08) Dec 2008, San Diego, CA, http://www.usenix.org/wiov08/ xxx SYSTOR 2009---The Israeli Experimental Systems Conference http://www.haifa.il.ibm.com/conferences/systor2009/ -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
