On 06/27/16 12:34, Christoffer Dall wrote:
> On Mon, Jun 27, 2016 at 11:47:18AM +0200, Ard Biesheuvel wrote:
>> So first of all, let me reiterate that I could only find a single
>> instance in QEMU where a PCI MMIO region is backed by host memory,
>> which is vga-pci.c. I wonder of there are any other occurrences, but
>> if there aren't any, it makes much more sense to prohibit PCI BARs
>> backed by host memory rather than spend a lot of effort working around
>> it.
>
> Right, ok. So Marc's point during his KVM Forum talk was basically,
> don't use the legacy VGA adapter on ARM and use virtio graphics, right?
The EFI GOP (Graphics Output Protocol) abstraction provides two ways for
UEFI applications to access the display, and one way for a runtime OS to
inherit the display hardware from the firmware (without OS native drivers).
(a) For UEFI apps:
- direct framebuffer access
- Blt() (block transfer) member function
(b) For runtime OS:
- direct framebuffer access ("efifb" in Linux)
Virtio-gpu lacks a linear framebuffer by design. Therefore the above
methods are reduced to the following:
(c) UEFI apps can access virtio-gpu with:
- GOP.Blt() member function only
(d) The runtime guest OS can access the virtio-gpu device as-inherited
from the firmware (i.e., without native drivers) with:
- n/a.
Given that we expect all aarch64 OSes to include native virtio-gpu
drivers on their install media, (d) is actually not a problem. Whenever
the OS kernel runs, we except to have no need for "efifb", ever. So
that's good.
The problem is (c). UEFI boot loaders would have to be taught to call
GOP.Blt() manually, whenever they need to display something. I'm not
sure about grub2's current status, but it is free software, so in theory
it should be doable. However, UEFI windows boot loaders are proprietary
*and* they require direct framebuffer access (on x86 at least); they
don't work with Blt()-only. (I found some Microsoft presentations about
this earlier.)
So, virtio-gpu is an almost universal solution for the problem, but not
entirely. For any given GOP, offering Blt() *only* (i.e., not exposing a
linear framebuffer) conforms to the UEFI spec, but some boot loaders are
known to present further requirements (on x86 anyway).
Thanks
Laszlo
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