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

From: Gleb Natapov
Date: Tue Dec 04 2012 - 06:47:58 EST


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.
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