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

[Igor Mammedov]

On Fri, 4 Sep 2015 12:04:41 +0200
Alexander Graf <address@hidden> 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()

> 
> Alex
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