On 09/16/16 15:44, Andrea Bolognani wrote:
On Fri, 2016-09-16 at 15:20 +0200, Pavel Hrdina wrote:
There is something I wanted to clarify with Laszlo: is virtio-gpu-pci ever going to be usable on other architectures such as x86_64? Maybe it already is? Because if that's the case, we'll want to be able to choose between virtio-vga and virtio-gpu-pci.
One solution would be to keep mapping model='virtio' to virtio-vga and create a new model='virtio-gpu' that maps to virtio-gpu-pci, then forbid aarch64 mach-virt guests to use model='virtio'. Or something like that, I'm not married to the idea, I just think it's something we should definitely think about before this ends up in a release.
I have some patches in my TODO branch that will rewrite the video device code. virtio-gpu-pci is usable also on other architectures but it lacks the VGA compatibility mode. In libvirt all primary video devices for x86 architecture have VGA mode. Currently we allow only QXL to be used as secondary video device and now with the virtio-gpu-pci it could be also used as secondary video device.
The solution would be simple, there is no need to add a new video model 'virtio-gpu', we will use the existing model 'virtio', but depending on architecture and also whether it's primary or secondary video device we will use appropriate device. We already do this for QXL.
I don't know much about video devices, so forgive me if I'm asking silly questions, but what is preventing you (on x86) from having virtio-vga as secondary video device?
The "VGA compatibility" stuff in virtio-vga is not just the linear framebuffer that lives in one of its MMIO BARs. VGA compatibility is much larger baggage; for example, it involves hard-coded ISA IO ports. Alex Williamson discusses VGA arbitration in the following blog post: http://vfio.blogspot.com/2014/08/whats-deal-with-vga-arbitration.html So the short answer is that "VGA compatibility" includes things that you mostly want to have *exactly one* instance of, in your x86 system. Zero instances of those things, and you might lose compatibility with some software; more than one instance of that stuff, and things get really messy with arbitration (hard-coded IO ports and MMIO ranges). So, even in the physical word, a multi-graphics-card x86 workstation usually has one primary VGA card (nowadays likely as an integrated device), carrying the legacy baggage; and then a number of secondary GPUs (that have none of that baggage).
Or virtio-gpu-pci as primary video device?
I described this earlier -- virtio-gpu-pci is more limited than virtio-vga as a primary graphics card, so wherever you can go with virtio-vga as primary, you'll do that. In aarch64/KVM guests however, virtio-vga is not an option, and the more limited (as primary) virtio-gpu-pci card is still better (much better) than nothing. Thanks Laszlo