Re: [RFC PATCH 0/6] kvm: Growable memory slot array

[Gleb Natapov <gleb@redhat.com>]

On Mon, Dec 03, 2012 at 04:39:05PM -0700, Alex Williamson wrote:
> Memory slots are currently a fixed resource with a relatively small
> limit.  When using PCI device assignment in a qemu guest it's fairly
> easy to exhaust the number of available slots.  I posted patches
> exploring growing the number of memory slots a while ago, but it was
> prior to caching memory slot array misses and thefore had potentially
> poor performance.  Now that we do that, Avi seemed receptive to
> increasing the memory slot array to arbitrary lengths.  I think we
> still don't want to impose unnecessary kernel memory consumptions on
> guests not making use of this, so I present again a growable memory
> slot array.
> 
> A couple notes/questions; in the previous version we had a
> kvm_arch_flush_shadow() call when we increased the number of slots.
> I'm not sure if this is still necessary.  I had also made the x86
> specific slot_bitmap dynamically grow as well and switch between a
> direct bitmap and indirect pointer to a bitmap.  That may have
> contributed to needing the flush.  I haven't done that yet here
> because it seems like an unnecessary complication if we have a max
> on the order of 512 or 1024 entries.  A bit per slot isn't a lot of
> overhead.  If we want to go more, maybe we should make it switch.
> That leads to the final question, we need an upper bound since this
> does allow consumption of extra kernel memory, what should it be?  A
This is the most important question :) If we want to have 1000s of
them or 100 is enough? Also what about changing kvm_memslots->memslots[]
array to be "struct kvm_memory_slot *memslots[KVM_MEM_SLOTS_NUM]"? It
will save us good amount of memory for unused slots.

> PCI bus filled with assigned devices can theorically use up to 2048
> slots (32 devices * 8 functions * (6 BARs + ROM + possibly split
> MSI-X BAR)).  For this RFC, I don't change the max, just make it
> grow up to 32 user slots.  Untested on anything but x86 so far.
> Thanks,
> 
> Alex
> ---
> 
> Alex Williamson (6):
>       kvm: Rename KVM_MEMORY_SLOTS -> KVM_USER_MEM_SLOTS
>       kvm: Make KVM_PRIVATE_MEM_SLOTS optional
>       kvm: Merge id_to_index into memslots
>       kvm: Move private memory slots to start of memslots array
>       kvm: Re-introduce memslots->nmemslots
>       kvm: Allow memory slots to grow
> 
> 
>  arch/ia64/include/asm/kvm_host.h    |    4 --
>  arch/ia64/kvm/kvm-ia64.c            |    6 +--
>  arch/powerpc/include/asm/kvm_host.h |    6 +--
>  arch/powerpc/kvm/book3s_hv.c        |    4 +-
>  arch/s390/include/asm/kvm_host.h    |    4 --
>  arch/x86/include/asm/kvm_host.h     |    8 ++-
>  arch/x86/include/asm/vmx.h          |    6 +--
>  arch/x86/kvm/vmx.c                  |    3 +
>  arch/x86/kvm/x86.c                  |   10 +++-
>  include/linux/kvm_host.h            |   20 +++++---
>  virt/kvm/kvm_main.c                 |   83 ++++++++++++++++++++++++-----------
>  11 files changed, 93 insertions(+), 61 deletions(-)

--
			Gleb.