[Bug 107561] 4.2 breaks PCI passthrough in QEMU/KVM

[bugzilla-daemon@bugzilla.kernel.org]

https://bugzilla.kernel.org/show_bug.cgi?id=107561

--- Comment #13 from schefister@gmail.com ---
Sorry for the delay. I couldn't check sooner.
Unfortunately the guest doesn't start with the patch applied. Nothing displayed
(no sync signal from passthrough card). Host dmesg doesn't show any errors.
Usually, even with slow guest I get the following on host dmesg when starting
VM:

...
VFIO - User Level meta-driver version: 0.3
kernel: vgaarb: device changed decodes:
PCI:0000:01:00.0,olddecodes=io+mem,decodes=io+mem:owns=io+mem
kernel: vfio_ecap_init: 0000:01:00.0 hiding ecap 0x1e@0x258
kernel: vfio_ecap_init: 0000:01:00.0 hiding ecap 0x19@0x900
kernel: pmd_set_huge: Cannot satisfy [mem 0xe0000000-0xe0200000] with a
huge-page mapping due to MTRR override.
- than some USB resets for keyboard passthrough -
kernel: kvm: zapping shadow pages for mmio generation wraparound
...and so on, the guest screen displays UEFI startup by now

With the patch applied it simly stops at the pmd_set_huge line, and nothing
further, no usb, no guest start, no error messages.

I also did follow up on the other tests.
Disabling host mtrrs one-by-one did not yield any results. Guest is still slow
even if disabling all the hosts mtrrs.
Tried nukeing kvm_mtrr_get_guest_memory_type as requested and started a live
distro. Guest did boot, but did not have a /proc/mtrr file. (Graphics scrolling
was slow, I guess on account of lack of cacheing, but otherwise guest was
speedy as normal) I saved some info from the guest, like dmesg, lspci -vvv, and
some sysfs files. You can find them in the same share as before.
https://drive.google.com/folderview?id=0B8ebX_WjVHnGNlN4eTEzU2xtMEk&usp=sharing

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