[vfio-users] regression from linux 4.0.4 to 4.2.0

[Kővágó Zoltán]

Hello,

I've been using vfio-pci to passthrough a GPU to virtual machine since 
some time now, and it worked great.  But this weekend I've finally had 
enough time to update kernel, and things are completely broken with the 
new kernel...

I've been using the ACS override patch (and a quick-and-dirty fix for 
multiple GOPs, but created a proper-ish patch yesterday, see 
http://article.gmane.org/gmane.linux.kernel.efi/6332 ), CSM disabled in 
UEFI and using OVMF virtual machines.  The motherboard is an ASRock Z87M 
Extreme4, with two PCI video cards (an NVidia GT640 (the primary card, 
used for linux), for which I almost had to beg at Gigabyte support to 
send an UEFI compatible VBIOS, and a GTX980 (secondary card, to 
passthrough)).  The integrated Intel GPU is diabled in UEFI settings. 
I'm not sure if it's supposed to work, but with 4.0.4 kernels it worked 
like a charm.

Now with 4.2.0, when I start qemu the monitor attached to the secondary 
card powers down, and then nothing happens, except qemu eating about 
150% cpu. I've started bisecting the kernel, and found out that

d69afbc6b1b5d0579f13d1a6339d952c4f60a9f4 KVM: MMU: fix decoding cache 
type from MTRR

is the culprit.  When mtrr is diabled, the old code returns 0xFF while 
the new returns MTRR_TYPE_UNCACHABLE.  I have absolutely no idea what 
the hell is going on here, but changing that return statement back 
solves the problem, until

b18d5431acc7a2fd22767925f3a6f597aa4bd29e KVM: x86: fix CR0.CD virtualization

If I comment out the if kvm_read_cd0 part it will work..  until 
4e241557fc1cb560bd9e77ca1b4a9352732a5427, which is a merge commit(!). 
I'm attaching a patch, it fixes the problem until 
f2ae45edbca7ba5324eef01719ede0151dc5cead for me.  But as I said earlier 
I have no freakin' idea what's going on here.

I have recompiled OVMF from svn yesterday evening, and have a recent-ish 
qemu master (with some audio related patches).  Tell me if you need any 
more information.

Thanks,
Zoltan
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