Re: [PATCH] hw/display/vga-isa: Fix migration of the isa-vga device

[Peter Maydell <peter.maydell@linaro.org>]

On Thu, 26 Mar 2026 at 12:04, Thomas Huth <thuth@redhat.com> wrote:
>
> On 26/03/2026 12.59, Peter Maydell wrote:
> > On Thu, 26 Mar 2026 at 11:35, Thomas Huth <thuth@redhat.com> wrote:
> >>
> >> From: Thomas Huth <thuth@redhat.com>
> >>
> >> QEMU currently crashes when migrating a guest that uses the
> >> isa-vga device as display. This happens because vga_isa_class_initfn()
> >> registers a vmsd for vmstate_vga_common that operates on VGACommonState.
> >> But the isa-vga device is derived from ISADevice, not from VGACommonState,
> >> so the migration code tries to fill in the data for VGACommonState to
> >> the memory that is a ISADevice instead, which is of cause causing trouble.
> >>
> >> We need an indirection here as it's also e.g. done in vga-pci.c, so
> >> that the migration data gets filled into the right location.
> >>
> >> While we're at it, also drop the "global_vmstate = true" here. Since
> >> migration was broken for this device during the last 15 years (!) anyway,
> >> we don't have to worry about maintaining backward compatibility with this
> >> switch for older versions of QEMU anymore.
> >
> > That means the only remaining users of global_vmstate = true are:
> >   * TYPE_VGA_MMIO : used only by the MIPS Jazz board, which is not
> >     versioned, so we are OK with migration compat breaks
> >   * TYPE_ISA_CIRRUS_VGA
> >
> > I don't suppose that migration is also currently broken for Cirrus ? :-)
>
> No, Cirrus seems to be fine. But I think we should likely do something like
> commit 1fcfdc435a3e25ab9037f6f7b8ab for the isa-cirrus-vga device, too, so
> we can at least get rid of this in a couple of years...

If we're going to do that it would be nice to get that in for 11.0.

I do notice that the Cirrus devices have their own vmstate handle
saving and restoring all the common VGA state, instead of using
vmstate_vga_common, and they miss out all the vbe entries at
the bottom...

-- PMM