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@xxxxxxxxxx> 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 ... > > 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. cheers, Gerd _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/virtualization
