Re: [Qemu-devel] [PATCH v5] xen: don't save/restore the physmap on VM save/restore

[Anthony PERARD]

On Wed, Mar 15, 2017 at 04:01:19PM +0000, Igor Druzhinin wrote:
> Saving/restoring the physmap to/from xenstore was introduced to
> QEMU majorly in order to cover up the VRAM region restore issue.
> The sequence of restore operations implies that we should know
> the effective guest VRAM address *before* we have the VRAM region
> restored (which happens later). Unfortunately, in Xen environment
> VRAM memory does actually belong to a guest - not QEMU itself -
> which means the position of this region is unknown beforehand and
> can't be mapped into QEMU address space immediately.
> 
> Previously, recreating xenstore keys, holding the physmap, by the
> toolstack helped to get this information in place at the right
> moment ready to be consumed by QEMU to map the region properly.
> 
> The extraneous complexity of having those keys transferred by the
> toolstack and unnecessary redundancy prompted us to propose a
> solution which doesn't require any extra data in xenstore. The idea
> is to defer the VRAM region mapping till the point we actually know
> the effective address and able to map it. To that end, we initially
> just skip the mapping request for the framebuffer if we unable to
> map it now. Then, after the memory region restore phase, we perform
> the mapping again, this time successfully, and update the VRAM region
> metadata accordingly.
> 
> Signed-off-by: Igor Druzhinin <address@hidden>

I've tried to migrate a guest with this patch, but once migrated, the
screen is black (via VNC, keyboard is working fine).

I haven't try to migrate a guest from QEMU without this patch to a QEMU
with it.

-- 
Anthony PERARD