On Mon, Mar 2, 2020 at 10:24 PM Gerd Hoffmann kraxel@redhat.com wrote:
On Mon, Mar 02, 2020 at 02:14:02PM -0800, Alistair Francis wrote:
On Fri, Feb 28, 2020 at 1:57 AM Gerd Hoffmann kraxel@redhat.com wrote:
On Thu, Feb 27, 2020 at 01:04:54PM -0800, Alistair Francis wrote:
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.
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