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Date: Tue, 17 Feb 2015 11:48:02 +0100
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH 1/2] vga: abort instead of shrinking memory
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To: Gerd Hoffmann <kraxel@redhat.com>
Cc: qemu-devel@nongnu.org

2015-02-17 11:37+0100, Gerd Hoffmann:
> On Di, 2015-02-17 at 11:29 +0100, Radim Kr=C4=8Dm=C3=A1=C5=99 wrote:
> > 2015-02-17 09:00+0100, Gerd Hoffmann:
> > > On Mo, 2015-02-16 at 22:23 +0100, Radim Kr=C4=8Dm=C3=A1=C5=99 wrote=
:
> > > > Automatic shrinking of vram_size leads to a segfault, because oth=
er
> > > > variables depend on being smaller and don't get shrinked.
> > >=20
> > > --verbose please.  Which other variables?
> >=20
> > I'm sorry, at least rom->surface0_area_size.
> > (It is sourced from qxl->vgamem_size.)
>=20
> Which command line triggers it?

The important subset is:
  -vga qxl -global qxl-vga.vgamem_mb=3D512

The segfault can then be triggered by any operation that dirties the
memory (pause for example).

> In theory qxl_init_ramsize() *should* make sure this can't happen ...
>=20
> I'd like to find & fix the bug instead of plugging an assert into some
> random place.

The bug happened because the init code is ovewriting variables, which
made the code unmanageable.  I added an assert, so we would fix the
callers.
Upper layers should also have no idea that our limit is 256, so we would
ideally return an error from vga_common_init() instead of silently
mangling sizes.