Re: [PATCH v5 0/8] KVM: MMU: fast zap all shadow pages

From: Takuya Yoshikawa
Date: Thu May 16 2013 - 10:36:33 EST

On Thu, 16 May 2013 20:17:45 +0800
Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx> wrote:

> Bechmark result:
> I have tested this patchset and the previous version that only zaps the
> pages linked on invalid slot's rmap. The benchmark is written by myself
> which has been attached, it writes large memory when do pci rom read.
> Host: Intel(R) Xeon(R) CPU X5690 @ 3.47GHz + 36G Memory
> Guest: 12 VCPU + 32G Memory
> Current code: This patchset Previous Version
> 2405434959 ns 2323016424 ns 2368810003 ns
> The interesting thing is, the previous version is slower than this patch,
> i guess the reason is that the former keeps lots of invalid pages in mmu
> which cause shadow page to be reclaimed due to used-pages > request-pages
> or host memory shrink.

This patch series looks very nice!

Minor issues may still need to be improved, but I really hope to see this
get merged during this cycle.

[for the future] Do you think that postponing some zapping/freeing of
obsolete(already invalidated) pages to make_mmu_pages_available() time
can improve the situation more? -- say, for big guests.

If accounting kept correct, make_mmu_pages_available() only needs to free
some obsolete pages instead of valid pages.


> Changlog:
> V5:
> 1): rename is_valid_sp to is_obsolete_sp
> 2): use lock-break technique to zap all old pages instead of only pages
> linked on invalid slot's rmap suggested by Marcelo.
> 3): trace invalid pages and kvm_mmu_invalidate_memslot_pages()
> 4): rename kvm_mmu_invalid_memslot_pages to kvm_mmu_invalidate_memslot_pages
> according to Takuya's comments.
> V4:
> 1): drop unmapping invalid rmap out of mmu-lock and use lock-break technique
> instead. Thanks to Gleb's comments.
> 2): needn't handle invalid-gen pages specially due to page table always
> switched by KVM_REQ_MMU_RELOAD. Thanks to Marcelo's comments.
> V3:
> completely redesign the algorithm, please see below.
> V2:
> - do not reset n_requested_mmu_pages and n_max_mmu_pages
> - batch free root shadow pages to reduce vcpu notification and mmu-lock
> contention
> - remove the first patch that introduce kvm->arch.mmu_cache since we only
> 'memset zero' on hashtable rather than all mmu cache members in this
> version
> - remove unnecessary kvm_reload_remote_mmus after kvm_mmu_zap_all
> * Issue
> The current kvm_mmu_zap_all is really slow - it is holding mmu-lock to
> walk and zap all shadow pages one by one, also it need to zap all guest
> page's rmap and all shadow page's parent spte list. Particularly, things
> become worse if guest uses more memory or vcpus. It is not good for
> scalability.
> * Idea
> KVM maintains a global mmu invalid generation-number which is stored in
> kvm->arch.mmu_valid_gen and every shadow page stores the current global
> generation-number into sp->mmu_valid_gen when it is created.
> When KVM need zap all shadow pages sptes, it just simply increase the
> global generation-number then reload root shadow pages on all vcpus.
> Vcpu will create a new shadow page table according to current kvm's
> generation-number. It ensures the old pages are not used any more.
> Then the invalid-gen pages (sp->mmu_valid_gen != kvm->arch.mmu_valid_gen)
> are zapped by using lock-break technique.
> Xiao Guangrong (8):
> KVM: MMU: drop unnecessary kvm_reload_remote_mmus
> KVM: MMU: delete shadow page from hash list in
> kvm_mmu_prepare_zap_page
> KVM: MMU: fast invalidate all pages
> KVM: x86: use the fast way to invalidate all pages
> KVM: MMU: make kvm_mmu_zap_all preemptable
> KVM: MMU: show mmu_valid_gen in shadow page related tracepoints
> KVM: MMU: add tracepoint for kvm_mmu_invalidate_memslot_pages
> KVM: MMU: zap pages in batch
> arch/x86/include/asm/kvm_host.h | 2 +
> arch/x86/kvm/mmu.c | 124 ++++++++++++++++++++++++++++++++++++++-
> arch/x86/kvm/mmu.h | 2 +
> arch/x86/kvm/mmutrace.h | 45 +++++++++++---
> arch/x86/kvm/x86.c | 9 +--
> 5 files changed, 163 insertions(+), 19 deletions(-)
> --
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Takuya Yoshikawa <takuya.yoshikawa@xxxxxxxxx>
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