Re: [Qemu-devel] [PATCH v4 2/2] loader: store FW CFG ROM files in RAM

[Laszlo Ersek <lersek@redhat.com>]

On 08/19/13 16:26, Michael S. Tsirkin wrote:
> ROM files that are put in FW CFG are copied to guest ram, by BIOS, but
> they are not backed by RAM so they don't get migrated.
> 
> Each time we change two bytes in such a ROM this breaks cross-version
> migration: since we can migrate after BIOS has read the first byte but
> before it has read the second one, getting an inconsistent state.
> 
> Future-proof this by creating, for each such ROM,
> an MR serving as the backing store.
> This MR is never mapped into guest memory, but it's registered
> as RAM so it's migrated with the guest.
> 
> Naturally, this only helps for -M 1.7 and up, older machine types
> will still have the cross-version migration bug.
> Luckily the race window for the problem to trigger is very small,
> which is also likely why we didn't notice the cross-version
> migration bug in testing yet.
> 
> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> ---
>  hw/core/loader.c    | 49 ++++++++++++++++++++++++++++++++++++++++++++++---
>  hw/i386/pc_piix.c   |  2 ++
>  hw/i386/pc_q35.c    |  2 ++
>  include/hw/loader.h |  1 +
>  4 files changed, 51 insertions(+), 3 deletions(-)

checked the v3->v4 diff in v4 0/2.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>