[Qemu-devel] AMD video card passthrough reset issues

["Lucio Andrés Illanes Albornoz" <l.illanes@gmx.de>]

Hello,

	I'm doing secondary VGA passthrough with an AMD Radeon R7 260X using QEMU v2.1.2 w/ KVM and VFIO on Debian v7.7 (wheezy) (qemu v2.1+dfsg-5~bpo70+1 from wheezy-backports) and kernel version 3.16.5 (from wheezy-backports as well) and Windows 8.1 Update 1 (x64) as the guest OS.

At present, rebooting the VM reproducibly has Windows fail to enable/start said video card upon bootup w/ an error code of 43, as seems to be the case w/ mostly everyone else running a comparable configuration; disabling/ejecting it before rebooting/powering down the VM from within the guest, as with everyone else, has proven to be a reliable mitigation. However, being that there are scenarios where this is either not feasible or impossible altogether, short of if done through a service or kernel-mode driver (and even then,) I had intended to investigate the causes behind this issue.

Unfortunately, the flu got to me first (so to speak.) I did notice that simply removing the PCI device in question and then causing a PCI bus (re)scan (both) through sysfs on the host in between VM reboots/power cycles is effectively equivalent to disabling it within the guest. Thus, I find myself wondering precisely what it is that does take place when doing so vs. when QEMU performs a `hot reset' through the corresponding interface in drivers/vfio/pci/; evidently, the difference must be of sufficient importance since the latter mechanism ends up leaving my video card unavailable for subsequent VM operation until the next host reboot.

I should very much appreciate any hints concerning whether it would be possible to have QEMU/VFIO perform whatever need be done itself or if it should be possible to have this be done by either itself.

Cheers
Lucio Andrés Illanes Albornoz