[Qemu-ppc] broken incoming migration

[Alexey Kardashevskiy]

Hi!

I found the migration broken on pseries platform, specifically, this patch
broke it:

f1c72795af573b24a7da5eb52375c9aba8a37972
migration: do not sent zero pages in bulk stage

The idea is not to send zero pages to the destination guest which is
expected to have 100% empty RAM.

However on pseries plaftorm the guest always has some stuff in the RAM as a
part of initialization (device tree, system firmware and rtas (?)) so it is
not completely empty. As the source guest cannot detect this, it skips some
pages during migration and we get a broken destination guest. Bug.

While the idea is ok in general, I do not see any easy way to fix it as
neither QEMUMachine::init nor QEMUMachine::reset callbacks has information
about whether we are about to receive a migration or not (-incoming
parameter) and we cannot move device-tree and system firmware
initialization anywhere else.

ram_bulk_stage is static and cannot be disabled from the platform
initialization code.

So what would the community suggest? Thanks!


-- 
Alexey