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Re: [Qemu-devel] vm state save/restore question

From: Alexander Graf
Subject: Re: [Qemu-devel] vm state save/restore question
Date: Tue, 19 Jun 2012 16:07:58 +0200

On 09.06.2012, at 13:34, Benjamin Herrenschmidt wrote:

> On Sat, 2012-06-09 at 20:53 +1000, Benjamin Herrenschmidt wrote:
>> Hi folks !
> (After some discussion with Andreas ...)
>> I'm looking at sorting out the state save/restore of target-ppc (which
>> means understanding in general how it works in qemu :-)
>> So far I've somewhat figured out that there's the "old way" where we
>> just provide a "bulk" save/restore function pair, and the "new way"
>> where we have nicely tagged field lists etc...
>> x86 seems to use the later for the CPU state, ppc is a mess and uses the
>> former with interesting incompatible format change depending on how qemu
>> is build :-) So I think that's one area I need to fix.
> Ok, so I'm told there are patches to convert ppc, I haven't seen them in
> my list archives, so if somebody has a pointer, please shoot, that will
> save me some work :-)
>  .../...
>> What I'd need is something in spapr that can be used to "resync" bits of
>> the cpu state with the external htab that gets run after everything is
>> loaded and before emulation restarts.
>> Any idea how to do that properly ? I suppose I could also try to iterate
>> all the vcpu's after loading the hash table & update the fields but not
>> only that's gross ... I also don't know how to do it :-)
> So I did an experiment using the "old style" save/restore (bad boy !)
> and got that part to work by just iterating the vcpu's.
> It's a bit nasty but it's the right way I think, ie, what we have here
> (the external hash table) is a global object under control/ownership of
> the platform code for which a pointer is cached in the CPU state (so the
> mmu emulation gets to it easily), so those cached pointers need to be
> updated in all CPUs when a new hash table is loaded/allocated.
> That leads to another question however... I need to add save/restore to
> a bunch more stuff such as the xics (interrupt controller), the various
> spapr devices, etc...
> So far the VMState stuff is all nice if you have fixed sized arrays.
> However I haven't quite found out the right way to use it for things
> like:
> - The hash table (mentioned above). This is just a big chunk of memory
> (it will routinely be 16M), so I really don't want to start iterating
> all elements, just a bulk load will do, and the size might actually be
> variable.
> - The XICS (interrupt controller). The actual size of the interrupt
> state array can vary (the number of interrupt sources can vary, it's
> fixed today by the machine code but I wouldn't rely too much on that and
> in any case, from the XICS driver perspective, it's not a constant, it's
> a variable it gets passed when initializing).
> So in both these cases, I need either code to control the save/load
> process (old style ? hard to hook into vmstate as far as I can tell) or
> maybe a way to describe the array so that the array size itself is a
> pointer to a variable (Andreas mentioned something along those lines).
> Is there any doco for that stuff btw ? I haven't seen anything
> detailed...

I'm sure Juan knows more there :)


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