Re: [Qemu-devel] [PATCH for-2.1 2/2] acpi: mark ACPI tables ROM blob as extend-able on migration

[Paolo Bonzini <pbonzini@redhat.com>]

On 04/11/2014 17:46, Michael S. Tsirkin wrote:
> On Tue, Nov 04, 2014 at 05:30:14PM +0100, Paolo Bonzini wrote:
>> On 03/11/2014 18:36, Dr. David Alan Gilbert wrote:
>>>   1) It's a block of data that's never mapped into the guests address
>>> space
>>>   2) It can change, but only at guest reset
>>>   3) Worst case is it can get upto about 2MB in size
>>>
>>> it's pretty marginal whether this thing should be a RAMBlock,
>>> it doesn't feel like normal RAM or ROM in most ways; but there
>>> again 2MB is getting a bit large for the device state; hmm.
>>
>> And also I think changing migration format gratuitously is bad.  We
>> decided to make these RAMs, which has some advantages and turned out to
>> have some possible disadvantages, but it's not a big deal.  They are
>> some kind of EPROM if you wish.
>>
>> The important point is that we can (and arguably _should_ since it keeps
>> us honest!) make these ACPI tables RAMBlocks fixed-size per machine
>> type.  See the patches I posted around late September/early October.
>> There is no need to support auto-fixing of the RAMBlock's sizes.
> 
> I'm not sure I buy that we should. ACPI bytecode can express
> identical interfaces in different ways. Even just recompiling
> ACPI from source can give you a different binary,
> same is true for a minor change in ACPI code.
> Migrating between two almost identical builds from qemu seems a
> very reasonable thing to do.

Yes, identical ACPI blocks are just a sufficient condition, not a
necessary one.  But it's very easy to enforce it, and it's what the
acpi-tables-test already checks.  It makes sense to me to stick with it.

(Regarding recompilation with a different iasl version, SSDT blocks are
simple enough that I think we can just build them in C code.  We're
already doing it for the much more complicated PCI bridge hotplug
interface.  BTW, can you pick up at least the patch to move the memory
hotplug device from SSDT to DSDT?).

Paolo