From: Crystal Kolipe <kolipe.c@exoticsilicon.com>
Subject: Re: sys/amd64: fallback to VGA text mode on headless systems
To: tech@openbsd.org
Date: Tue, 27 Jan 2026 12:18:50 +0000

On Tue, Jan 27, 2026 at 11:31:50AM +0000, Stuart Henderson wrote:
> On 2026/01/27 12:05, Kirill A. Korinsky wrote:
> > On Tue, 27 Jan 2026 10:21:02 +0100,
> > Crystal Kolipe <kolipe.c@exoticsilicon.com> wrote:
> > > Whilst it fixes your machine, how confident can we be that this is harmless on
> > > others?
> > > 
> > > It's perfectly valid to have no graphics hardware at all and run entirely from
> > > a serial console, so this could plausibly break machines that are currently
> > > working just fine.
> > > 
> > 
> > Well, here my assumption that on amd64, both the VGA legacy memory region
> > and I/O ports is still reserved and not used.
> 
> if my reading is correct then I think that is ok in this case (but I'm
> not 100%) ..
> 
> boot> machine mem
> Region 0: type 1 at 0x0 for 639KB
> Region 1: type 2 at 0x9fc00 for 1KB
> Region 2: type 2 at 0xf0000 for 64KB
> Region 3: type 1 at 0x100000 for 2078296KB
> Region 4: type 2 at 0x7ee96000 for 17832KB
> Region 5: type 2 at 0xf8000000 for 65536KB
> Region 6: type 2 at 0xfec10000 for 4KB
> Region 7: type 2 at 0xfed40000 for 20KB
> Low ram: 639KB  High ram: 2078296KB
> Total free memory: 2078935KB

The VGA memory space is certainly marked as reserved.

But 'reserved' doesn't mean that writing arbitrary values to those addresses
is harmless.