Re: [Qemu-devel] VFIO-VGA Issue

[Gleb Natapov]

On Tue, Apr 09, 2013 at 04:17:48PM -0600, Alex Williamson wrote:
> On Tue, 2013-04-09 at 11:18 -0600, Alex Williamson wrote:
> > On Tue, 2013-04-09 at 08:22 -0400, address@hidden wrote:
> > > Hi!
> > > 
> > > I've tried passing through my ASUS Radeon HD7750, but qemu just hogs 100%
> > > of one core and stops. Since the display didn't light up, I couldn't see
> > > how far Win 7 boot went. For a Linux guest I tried redirecting kernel
> > > messages to serial console, but there was nothing (qemu stops even before
> > > kernel start?).
> > > 
> > > Also, this GPU is kinda troublesome: it doesn't work with the proprietary
> > > driver if VT-d is enabled in BIOS (boot process stops with "dmar: DRHD:
> > > handling fault status reg 3"). The GPU works with the FOSS driver, though.
> > > Could this somehow be related to the passthrough issue?
> > > 
> > > Anyway, VT-d in pci-assign mode works for network adapters on this host.
> > > It runs kernel 3.9.0-rc5 (with vfio-vga enabled), qemu git from yesterday,
> > > and seabios 1.7.2. The host has no driver installed for the GPU. Before
> > > assiging it to qemu I'd run "echo '1002 683f' >
> > > /sys/bus/pci/drivers/vfio-pci/new_id" and the same for the associated HDMI
> > > device. Command line:
> > > qemu-kvm -machine q35,accel=kvm \
> > >    -cpu host -smp 4 -m 2G \
> > >    -vga none -display none \
> > >    -kernel /kvm/kernel-3.7.8 -append 'root=/dev/vda console=ttyS0' \
> > >    -drive
> > > index=0,media=disk,cache=none,if=virtio,aio=native,file=/dev/vg3/gpu \
> > >    -device vfio-pci,host=01:00.0,x-vga=on \
> > >    -device vfio-pci,host=01:00.1 \
> > >    -serial telnet:127.0.0.1:52990,server
> > 
> > You should always at least get monitor sync if it's going to work, but
> > you may have to reboot the host between each attempt.  I've been doing
> > most of the VGA development on an AMD box and swapping over to an Intel
> > system reminds me why.  Try removing the ",accel=kvm -cpu host -smp 4"
> > pieces of your command an try again.  If that works, it means there are
> > still broken bits of real mode handling in KVM.  My HD7850 hangs on an
> > Intel host the same way you describe, but posts if I disable KVM
> > support.  Thanks,
> 
> Gleb,
> 
> If I run trace-cmd while I'm in this hung state with KVM enabled, the
> log is filled with:
> 
>  qemu-system-x86-9569  [000] 10027.806836: kvm_emulate_insn:     c0000:b2d3: 
> ff 7d 00
> 
> I hope this means more to you than it does to me ;)  Based on the
> segment register we're executing in the VGA BIOS for the assigned VGA
> device.  Let me know what more I can provide to help debug.  Thanks,
> 
This is wrong encoding. Such instruction does not exists. Can you see
what happens before this emulation starts to happen.

--
                        Gleb.