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

[Thomas Huth <thuth@redhat.com>]

On 07/09/15 16:31, Igor Mammedov wrote:
> On Fri, 4 Sep 2015 12:04:41 +0200
> Alexander Graf <agraf@suse.de> wrote:
> 
>>
>>
>> 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?
> 
> KVM switched memslots allocation to kvm_kvzalloc(), so it would fallback to vmalloc
>  commit 744961341d472db6272ed9b42319a90f5a2aa7c4
>  kvm: avoid page allocation failure in kvm_set_memory_region()

Good hint, thanks for pointing that out! ... so increasing the array
size should not cause too much trouble :-)

 Thomas