Re: [Qemu-ppc] KVM memory slots limit on powerpc

[Thomas Huth]

On 04/09/15 12:07, Christian Borntraeger wrote:
> Am 04.09.2015 um 12:04 schrieb Alexander Graf:
>>
>>
>> On 04.09.15 11:59, Christian Borntraeger wrote:
>>> Am 04.09.2015 um 11:35 schrieb Thomas Huth:
>>>>
>>>>  Hi all,
>>>>
>>>> now that we get memory hotplugging for the spapr machine on qemu-ppc,
>>>> too, it seems like we easily can hit the amount of KVM-internal memory
>>>> slots now ("#define KVM_USER_MEM_SLOTS 32" in
>>>> arch/powerpc/include/asm/kvm_host.h). For example, start
>>>> qemu-system-ppc64 with a couple of "-device secondary-vga" and "-m
>>>> 4G,slots=32,maxmem=40G" and then try to hot-plug all 32 DIMMs ... and
>>>> you'll see that it aborts way earlier already.
>>>>
>>>> The x86 code already increased the amount of KVM_USER_MEM_SLOTS to 509
>>>> already (+3 internal slots = 512) ... maybe we should now increase the
>>>> amount of slots on powerpc, too? Since we don't use internal slots on
>>>> POWER, would 512 be a good value? Or would less be sufficient, too?
>>>
>>> When you are at it, the s390 value should also be increased I guess.
>>
>> That constant defines the array size for the memslot array in struct kvm
>> which in turn again gets allocated by kzalloc, so it's pinned kernel
>> memory that is physically contiguous. Doing big allocations can turn
>> into problems during runtime.
>>
>> So maybe there is another way? Can we extend the memslot array size
>> dynamically somehow? Allocate it separately? How much memory does the
>> memslot array use up with 512 entries?
> 
> Maybe some rcu protected scheme that doubles the amount of memslots for
> each overrun? Yes, that would be good and even reduce the footprint for
> systems with only a small number of memslots.

Seems like Alex Williamson already posted a patchset for growable
memslots a couple of years ago:

 http://www.spinics.net/lists/kvm/msg50491.html

But I didn't quite spot the result in that thread why it never has been
included upstream. Alex (W.), do you remember the outcome?

 Thomas