Re: [libvirt] [PATCH v4 3/3] hw/vfio/display: add ramfb support
On Wed, Jun 13, 2018 at 01:50:47PM -0600, Alex Williamson wrote:
On Wed, 13 Jun 2018 10:41:49 +0200
Gerd Hoffmann <kraxel(a)redhat.com> wrote:
> So we have a boot display when using a vgpu as primary display.
>
> Use vfio-pci-ramfb instead of vfio-pci to enable it.
Using a different device here seems like it almost guarantees a very
complicated path to support under libvirt. What necessitates this
versus a simple ramfb=on option to vfio-pci?
Well, it's simliar to qxl vs. qxl-vga. It's not qxl,vga={on,off} and
libvirt has no problems to deal with that ...
Another more technical reason is (again) hotplug. ramfb needs an fw_cfg
entry for configuration, and fw_cfg entries can't be hotplugged. So
hotplugging vfio-pci with ramfb=on isn't going to fly. So we need a
separate device with hotplug turned off.
I'm also not sure I understand the usage model, SeaBIOS and OVMF
know
how to write to this display, but it seems that the guest does not.
Yes.
I suppose in the UEFI case runtime services can be used to continue
writing this display,
Yes.
but BIOS doesn't have such an option, unless we're somehow
emulating
VGA here.
vgabios support is in the pipeline, including text mode emulation (at
vgabios level, direct access to vga window @ 0xa0000 doesn't work).
So for UEFI, I can imagine this
covers us from power on through firmware boot and up to guest drivers
initializing the GPU (assuming the vGPU supports a kernel mode driver,
does NVIDIA?),
Yes. Shouldn't matter whenever the driver is kernel or userspace.
but for BIOS it seems we likely still have a break from
the bootloader to GPU driver initialization.
Depends. vgacon (text mode console) doesn't work. fbcon @ vesafb works.
For instance, what driver
is used to draw the boot animation (or blue screen) on SeaBIOS Windows
VM?
Windows depends on vgabios for that and it works fine.
I'm assuming that this display and the vGPU display are one in
the
same, so there's some cut from one to the other.
Yes. If the vfio query plane ioctl reports a valid guest video mode
configuration the vgpu display will be used, ramfb otherwise.
cheers,
Gerd