On Mon, Mar 2, 2020 at 10:24 PM Gerd Hoffmann [off-list ref] wrote:

On Mon, Mar 02, 2020 at 02:14:02PM -0800, Alistair Francis wrote:

quoted

On Fri, Feb 28, 2020 at 1:57 AM Gerd Hoffmann [off-list ref] wrote:

quoted

On Thu, Feb 27, 2020 at 01:04:54PM -0800, Alistair Francis wrote:

quoted

The QEMU model for the Bochs display has no VGA memory section at offset
0x400 [1]. By writing to this register Linux can create a write to
unassigned memory which depending on machine and architecture can result
in a store fault.

I don't see any reference to this address at OSDev [2] or in the Bochs
source code.

Removing this write still allows graphics to work inside QEMU with
the bochs-display.

It's not that simple.  The driver also handles the qemu stdvga (-device
VGA, -device secondary-vga) which *does* need the vga port write.
There is no way for the guest to figure whenever the device is
secondary-vga or bochs-display.

So how about fixing things on the host side?  Does qemu patch below
help?

That patch looks like it will fix the problem, but it doesn't seem
like the correct fix. I would rather avoid adding a large chunk of
dummy I/O to handle the two devices.

It's just a single handler for the parent mmio region, so we have a
defined default action instead of undefined behavior.

Patch just posted to qemu-devel, lets see what others think ...

Thanks, let's wait and see what happens.

quoted

quoted

Maybe another possible approach is to enable/disable vga access per
arch.  On x86 this doesn't cause any problems.  I guess you are on
risc-v?

I would prefer this option. I do see this on RISC-V, but I suspect the
issue will appear on other architectures (although how they handle I/O
failures in QEMU is a different story).

Can I just do the VGA write if x86?

I know ppc needs it too.  Not sure about other architectures.  I'd
suggest to do it the other way around: blacklist known-problematic
archs.

Argh, that is a little uglier. Let's circle back after receiving
feedback on the QEMU patch.

Alistair

cheers,
  Gerd