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

[Paolo Bonzini <pbonzini@redhat.com>]

Il 19/08/2013 19:40, Michael S. Tsirkin ha scritto:
> On Mon, Aug 19, 2013 at 07:28:12PM +0200, Paolo Bonzini wrote:
>> Il 19/08/2013 16:26, Michael S. Tsirkin ha scritto:
>>> 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>
>>
>> This doesn't really help much unless we support migration of
>> different-sized RAM regions, does it?
> 
> It does because RAM blocks are multiples of target pages, so the size
> won't change all that much. This is more or less similar to what happens
> e.g. with BIOS (aligned to power of two).
> It worked for BIOS for a while.

Yeah, it should work.

On one hand there are many different files, so many different points of
failures.  On the other hand most of the files are small, so the
almost-4 KiB legroom is not bad.

> No, migration fails if block size does not match.

Good.

> Well they protect us against minor changes which are
> IMO more likely than major changes.
> 
> We can add code to allow ram block size changes on top.
> I'm not sure it's a requirement right now.

No, it's not---thanks for the clarifications.

Paolo