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

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(a)redhat.com&gt; Cc: Drew Jones <drjones(a)redhat.com&gt; Cc: Marc-André Lureau <marcandre.lureau(a)redhat.com&gt; Suggested-by: Marc-André Lureau <marcandre.lureau(a)redhat.com&gt; Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1372901 Signed-off-by: Laszlo Ersek <lersek(a)redhat.com&gt;