Re: [Qemu-devel] [PATCH 2/3] ramfb enhancement

[Gerd Hoffmann]

On Fri, May 10, 2019 at 02:41:36PM +0800, Hou Qiming wrote:
> > Only allow one resolution change per guest boot, which prevents a
> 
> > > crash when the guest writes garbage to the configuration space (e.g.
> > > when rebooting).
> >
> > Hmm?  Did you see that happen in practice?
> > It is not easy to write to fw_cfg by accident ...
> >
> >
> Yes, this does happen in practice. It's observed in KVMGT setups by another
> github user and me, when the guest Intel driver loads or when the guest
> reboots. Link:
> https://github.com/intel/gvt-linux/issues/23#issuecomment-483651476
> 
> Now that you mentioned it, I start to feel that it's not accidental. A
> closer look at the "garbage" in that post shows that the overwriting
> content are valid resolution values in the wrong endian. It could be a
> misguided attempt to "resize ramfb" by the guest Intel driver.

Hmm.  The intel driver certainly isn't supposed to do that ...

So, allow writing only once might be a good idea, to make clear this
*really* is meant to be used by the firmware only, for a boot display.

cheers,
  Gerd