[PATCH] drm/bochs: Remove vga write

[Gerd Hoffmann]

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 at 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 ...

> > 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