On 20/08/19 07:22, Peter Xu wrote: > On Mon, Aug 12, 2019 at 09:45:27AM +0200, Peter Xu wrote: >> This is a RFC series. >> >> The VT-d code has some defects, one of them is that we cannot detect >> the misuse of vIOMMU and vfio-pci early enough. >> >> For example, logically this is not allowed: >> >> -device intel-iommu,caching-mode=off \ >> -device vfio-pci,host=05:00.0 >> >> Because the caching mode is required to make vfio-pci devices >> functional. >> >> Previously we did this sanity check in vtd_iommu_notify_flag_changed() >> as when the memory regions change their attributes. However that's >> too late in most cases! Because the memory region layouts will only >> change after IOMMU is enabled, and that's in most cases during the >> guest OS boots. So when the configuration is wrong, we will only bail >> out during the guest boots rather than simply telling the user before >> QEMU starts. >> >> The same problem happens on device hotplug, say, when we have this: >> >> -device intel-iommu,caching-mode=off >> >> Then we do something like: >> >> (HMP) device_add vfio-pci,host=05:00.0,bus=pcie.1 >> >> If at that time the vIOMMU is enabled in the guest then the QEMU >> process will simply quit directly due to this hotplug event. This is >> a bit insane... >> >> This series tries to solve above two problems by introducing two >> sanity checks upon these places separately: >> >> - machine done >> - hotplug device >> >> This is a bit awkward but I hope this could be better than before. >> There is of course other solutions like hard-code the check into >> vfio-pci but I feel it even more unpretty. I didn't think out any >> better way to do this, if there is please kindly shout out. >> >> Please have a look to see whether this would be acceptable, thanks. > > Any more comment on this?
No problem from me, but I wouldn't mind if someone else merged it. :) Paolo
