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[Qemu-devel] AMD video card passthrough reset issues

From: Lucio Andrés Illanes Albornoz
Subject: [Qemu-devel] AMD video card passthrough reset issues
Date: Tue, 2 Dec 2014 13:53:11 +0100


        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.

Lucio Andrés Illanes Albornoz

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