Device pass-through

[Gordon Messmer <yinyang@eburg.com>]

I'm going to summarize my questions first and write the long explanation 
afterward...

1: What happens when Linux rejects the aperture allocated by the BIOS 
for an AMD IOMMU?
2: What is unsafe about the allow_unsafe_assigned_interrupts option? 
What are the potential consequences of its use?
3: How can I get the ROM from a graphics card?
4: What do these errors mean, and is there any way I can correct the 
problem?
> create_userspace_phys_mem: File exists
> assigned_dev_iomem_map: Error: create new mapping failed

My goal is to replace two machines that I previously used with one 
machine and a KVM guest.  I previously had one system that was always on 
and ran a stable OS, and a second system for my desktop.  I could afford 
to experiment or format the desktop system without interrupting access 
to my data, all of which is on the stable system.

I'd like to use KVM to accomplish something more or less equivalent by 
creating a guest and passing the keyboard, mouse, audio device, and a 
video card to it.  To this end, I have a new AMD system with a Gigabyte 
GA-970A-D3 motherboard and two Radeon video cards.  Based on my reading, 
I should probably be able to pass one of those to the guest as a PCI 
device.  It isn't in use by the host, and I'm not trying to make it the 
guest's primary display, so I don't have to worry about early boot 
output.  The guest will still have a VNC console as its primary display. 
  I'm mostly there now.  I can pass through the host's keyboard, mouse, 
and audio device.

Naturally, there are a couple of challenges.  First, the IOMMU on this 
board is a little wonky.  The kernel currently reports:

> kernel: Checking aperture...
> kernel: No AGP bridge found
> kernel: Node 0: aperture @ 20000000 size 32 MB
> kernel: Aperture pointing to e820 RAM. Ignoring.
> kernel: Your BIOS doesn't leave a aperture memory hole
> kernel: Please enable the IOMMU option in the BIOS setup
> kernel: This costs you 64 MB of RAM
> kernel: Mapping aperture over 65536 KB of RAM @ 20000000
> kernel: PM: Registered nosave memory: 0000000020000000 -
>         0000000024000000
> kernel: PCI-DMA: Using software bounce buffering for IO (SWIOTLB)
> kernel: Placing 64MB software IO TLB between ffff880037ff0000 -
>         ffff88003bff0000
> kernel: software IO TLB at phys 0x37ff0000 - 0x3bff0000

additionally:

> kernel: BIOS-provided physical RAM map:
> kernel: BIOS-e820: 0000000000000000 - 0000000000098800 (usable)
> kernel: BIOS-e820: 000000000009f800 - 00000000000a0000 (reserved)
> kernel: BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
> kernel: BIOS-e820: 0000000000100000 - 00000000bfda0000 (usable)
> kernel: BIOS-e820: 00000000bfda0000 - 00000000bfdd1000 (ACPI NVS)
> kernel: BIOS-e820: 00000000bfdd1000 - 00000000bfe00000 (ACPI data)
> kernel: BIOS-e820: 00000000bfe00000 - 00000000bff00000 (reserved)
> kernel: BIOS-e820: 00000000e0000000 - 00000000f0000000 (reserved)
> kernel: BIOS-e820: 00000000fec00000 - 0000000100000000 (reserved)
> kernel: BIOS-e820: 0000000100000000 - 0000000440000000 (usable)
> ...
> kernel: ACPI: IVRS 00000000bfddff00 000D0
>         (v01  AMD     RD890S 00202031 AMD  00000000)

Initially the kernel reported "AMD-Vi disabled by default: pass 
amd_iommu=on to enable"  After booting with that option, I seem to be 
able to boot guests with devices passed through.  That leads me to the 
first question above.  How bad is the state of the IOMMU on this board? 
  Where should the IVRS be mapped instead?

In addition to the amd_iommu boot option, I had to add:
options kvm allow_unsafe_assigned_interrupts=1
in order to pass through the audio device.  That leads me to the second 
question.  What's unsafe about this?

When I try to boot the guest and pass in the video card, I get a warning 
about the ROM:

> pci-assign: Cannot read from host /sys/bus/pci/devices/0000:05:00.0/rom
>         Device option ROM contents are probably invalid (check dmesg).
>         Skip option ROM probe with rombar=0, or load from file with
>         romfile=
> create_userspace_phys_mem: File exists
> assigned_dev_iomem_map: Error: create new mapping failed

Third question, when I try to read that path, I get an IO error and the 
kernel says "pci 0000:05:00.0: Invalid ROM contents".  Is there any 
other way to get the ROM from the card?

I've tried the other option.  I added <rom bar='off'/> to the libvirt 
definition for this guest.  When I try to boot the guest with that 
option, the kernel doesn't add anything to dmesg that looks relevant, 
but the libvirt log says:

> LC_ALL=C PATH=/sbin:/usr/sbin:/bin:/usr/bin QEMU_AUDIO_DRV=none /usr/libexec/qemu-kvm -S -M rhel6.2.0 -enable-kvm -m 4096 -smp 4,sockets=4,cores=1,threads=1 -name herald -uuid 1929f6a4-b2dd-a4fd-fe07-747b7fdf9fae -nodefconfig -nodefaults -chardev socket,id=charmonitor,path=/var/lib/libvirt/qemu/herald.monitor,server,nowait -mon chardev=charmonitor,id=monitor,mode=control -rtc base=utc -no-shutdown -drive file=/dev/mapper/VolGroup-lv_vm_herald,if=none,id=drive-virtio-disk0,format=raw,cache=none,aio=native -device virtio-blk-pci,bus=pci.0,addr=0x4,drive=drive-virtio-disk0,id=virtio-disk0,bootindex=1 -netdev tap,fd=25,id=hostnet0,vhost=on,vhostfd=26 -device virtio-net-pci,netdev=hostnet0,id=net0,mac=52:54:00:86:e0:ea,bus=pci.0,addr=0x3 -chardev pty,id=charserial0 -device isa-serial,chardev=
 charserial0,id=serial0 -usb -device usb-tablet,id=input0 -vnc 127.0.0.1:0 -vga cirrus -device pci-assign,host=00:14.2,id=hostdev0,configfd=27,bus=pci.0,addr=0x5 -device pci-assign,host=05:00
.0,id=hostdev1,configfd=28,bus=pci.0,addr=0x7,rombar=0 -device virtio-balloon-pci,id=balloon0,bus=pci.0,addr=0x6
> char device redirected to /dev/pts/1
> create_userspace_phys_mem: File exists
> assigned_dev_iomem_map: Error: create new mapping failed

So, my last question is what do the last two lines mean?