[libvirt] [PATCH] qemu: map "virtio" video model to "virt" machtype correctly (arm/aarch64)

[Martin Kletzander]

On Fri, Sep 16, 2016 at 06:04:46AM +0200, Laszlo Ersek wrote:
>Most of QEMU's PCI display device models, such as:
>
>  libvirt video/model/@type  QEMU -device
>  -------------------------  ------------
>  cirrus                     cirrus-vga
>  vga                        VGA
>  qxl                        qxl-vga
>  virtio                     virtio-vga
>
>come with a linear framebuffer (sometimes called "VGA compatibility
>framebuffer"). This linear framebuffer lives in one of the PCI device's
>MMIO BARs, and allows guest code (primarily: firmware drivers, and
>non-accelerated OS drivers) to display graphics with direct memory access.
>
>Due to architectural reasons on aarch64/KVM hosts, this kind of
>framebuffer doesn't / can't work in
>
>  qemu-system-(arm|aarch64) -M virt
>
>machines. Cache coherency issues guarantee a corrupted / unusable display.
>The problem has been researched by several people, including kvm-arm
>maintainers, and it's been decided that the best way (practically the only
>way) to have boot time graphics for such guests is to consolidate on
>QEMU's "virtio-gpu-pci" device.
>
>>From <https://bugzilla.redhat.com/show_bug.cgi?id=1195176>, libvirt
>supports
>
>  <devices>
>    <video>
>      <model type='virtio'/>
>    </video>
>  </devices>
>
>but libvirt unconditionally maps @type='virtio' to QEMU's "virtio-vga"
>device model. (See the qemuBuildDeviceVideoStr() function and the
>"qemuDeviceVideo" enum impl.)
>
>According to the above, this is not right for the "virt" machine type; the
>qemu-system-(arm|aarch64) binaries don't even recognize the "virtio-vga"
>device model (justifiedly). Whereas "virtio-gpu-pci", which is a pure
>virtio device without a compatibility framebuffer, is available, and works
>fine.
>
>(The ArmVirtQemu ("AAVMF") platform of edk2 -- that is, the UEFI firmware
>for "virt" -- supports "virtio-gpu-pci", as of upstream commit
>3ef3209d3028. See
><https://tianocore.acgmultimedia.com/show_bug.cgi?id=66>.)
>
>Override the default mapping of "virtio", from "virtio-vga" to
>"virtio-gpu-pci", if qemuDomainMachineIsVirt() evaluates to true.
>
>Cc: Andrea Bolognani <abologna at redhat.com>
>Cc: Drew Jones <drjones at redhat.com>
>Cc: Marc-André Lureau <marcandre.lureau at redhat.com>
>Suggested-by: Marc-André Lureau <marcandre.lureau at redhat.com>
>Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1372901
>Signed-off-by: Laszlo Ersek <lersek at redhat.com>

Very nicely written, makes sense and since virtio-vga didn't work for
virt machines, there is no problem with changing it.

One small nit though, syntax-check will complain that one of the lines
in the .args file is longer, you can either use 'tests/test-wrap-argv.pl
--in-place tests/qemuxml2argvdata/qemuxml2argv-aarch64-video-virtio-gpu-pci.args'
or just squash this diff in, whatever you find easier:

diff --git i/tests/qemuxml2argvdata/qemuxml2argv-aarch64-video-virtio-gpu-pci.args w/tests/qemuxml2argvdata/qemuxml2argv-aarch64-video-virtio-gpu-pci.args
index 56dbdfb66fa2..76ee977a3ca2 100644
--- i/tests/qemuxml2argvdata/qemuxml2argv-aarch64-video-virtio-gpu-pci.args
+++ w/tests/qemuxml2argvdata/qemuxml2argv-aarch64-video-virtio-gpu-pci.args
@@ -20,7 +20,7 @@ QEMU_AUDIO_DRV=none \
 addr=0x1 \
 -device ioh3420,port=0x9,chassis=2,id=pci.2,bus=pcie.0,multifunction=on,\
 addr=0x1.0x1 \
--device virtio-net-pci,vlan=0,id=net0,mac=52:54:00:73:34:53,bus=pci.1,\
-addr=0x0,bootindex=1 \
+-device virtio-net-pci,vlan=0,id=net0,mac=52:54:00:73:34:53,bus=pci.1,addr=0x0,\
+bootindex=1 \
 -net user,vlan=0,name=hostnet0 \
 -device virtio-gpu-pci,id=video0,bus=pci.2,addr=0x0
--

Other than that, with that small thing fixed:

Acked-by: Martin Kletzander <mkletzan at redhat.com>
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