Re: [Qemu-devel] VFIO-VGA Issue

[Alex Williamson]

On Wed, 2013-04-10 at 12:01 +0300, Gleb Natapov wrote:
> 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.

Yeah, it seems like b2d3 is a bad offset, if I align the code at d2 or
d0 gdb is able to disassemble it.  I'm not having much luck with
breakpoints or stepping, but I'll keep fiddling with it to try to figure
out how we're getting here.  Thanks,

Alex