2.6.0 hangs linux vm using vfio for pci passthrough of graphics card

Not a duplicate of my old bug 1488363

qemu version 2.5.1 works fine
qemu version 2.6.0 fails

seabios 1.9.2-1

using kernel 4.5.5 with grsecurity

I built using the arch packaging tools, but commented out all the patch code, so it should be vanilla.

The problem is just that I start a Linux vm using either my radeon R7 260x or radeon HD 6770, and with qemu 2.6.0, it looks normal until after the grub menu, and then the screen looks broken (with mostly black, and some pixely junk spread horizontally in a few places on the screen... first we thought maybe the monitor died). I'm not sure if it's before or only at the moment where the screen resolution changes (I could check that or record it on request). Also, the VM is not pingable and does not respond to "system_powerdown" on qemu monitor.

However, the same setup works fine with windows 8. And it works fine without graphics cards passed through. A usb controller passed through works fine too.


And then I ran a bisect...

        2d82f8a3cdb276bc3cb92d6f01bf8f66bf328d62 is the first bad commit
        commit 2d82f8a3cdb276bc3cb92d6f01bf8f66bf328d62
        Author: Alex Williamson <email address hidden>
        Date:   Thu Mar 10 09:39:08 2016 -0700

            vfio/pci: Convert all MemoryRegion to dynamic alloc and consistent functions
            
            Match common vfio code with setup, exit, and finalize functions for
            BAR, quirk, and VGA management.  VGA is also changed to dynamic
            allocation to match the other MemoryRegions.
            
            Signed-off-by: Alex Williamson <email address hidden>

        :040000 040000 0acfd49b6ecae780b6f52a34080ecec6b3ec3672 e0cfdadede08f553463c0b23931eda81107f41b8 M      hw
        
then confirm it by reverting that commit
        git checkout v2.6.0
        git revert 2d82f8a3cdb276bc3cb92d6f01bf8f66bf328d62
        git mergetool -t kdiff3
            "select all from C", save
            not sure if this is the right way to do this...but it compiles and works (bug fixed)
        git commit -m "revert 2d82f8a3cdb276bc3cb92d6f01bf8f66bf328d62 resolve conflicts"

And that 2.6.0 build with that one patch reverted works fine.