Re: [PATCH v8 00/11] KVM: MMU: fast zap all shadow pages

[Gleb Natapov]

On Fri, May 31, 2013 at 08:36:19AM +0800, Xiao Guangrong wrote:
> Hi Gleb, Paolo, Marcelo,
> 
> I have putted the potential controversial patches to the latter that are
> patch 8 ~ 10, patch 11 depends on patch 9. Other patches are fully reviewed,
> I think its are ready for being merged. If not luck enough, further discussion
> is needed, could you please apply that patches first? :)
> 
> Thank you in advance!
> 
> Some points are raised during discussion but missed in this version:
> 1) Gleb's idea that skip obsolete pages in the hast list walker
> 
>    Unfortunately, it is not safe. There has a window between updating
>    valid-gen and reloading mmu, in that window, the obsolete page can
>    be used by vcpu, but the guest page table fail to be write-protected
>    (since the obsolete page is skipped in mmu_need_write_protect()).
> 
Can you elaborate on how this can happen. valid_gen is updated under
mmu_lock and reloading of mmus happens under the same lock, so for all
other vcpus this should look like atomic thing.

>    Instead, we can only skip the zapped-obsolete page
>    (is_obsolete_sp(sp) && sp->role.invalid)), the current code has already
>    skip them but put the comment around the hash list walker to warn the
>    further development.
> 
> 2) Marcelo's comment that obsolete pages can cause the number of shadow page
>    greater than the n_max_mmu_pages
> 
>    I am not sure this is really a problem, it only exists in the really tiny
>    window and the page-reclaim path are able to handle the obsolete pages.
>    Furthermore, we can properly reduce n_max_mmu_pages to make that window
>    more tiny.
> 
>    Anyway, like commit 5d21881432 shows that "the mmu counters are for
>    beancounting purposes only", maybe that window is allowed.
> 
> Changlog:
> V8:
>   1): add some comments to explain FIFO around active_mmu_list address
>       Marcelo's comments.
> 
>   2): the page-reclaim path may fail to free zapped-obsolete pages pointed
>       out by Marcelo, the patchset fixes it by listing all zapped obsolete
>       pages on a global list, always free page from that list first.
> 
>   3): address Marcelo's suggestion to move the "zap pages in batch" patch
>       to the latter.
> 
>   4): drop the previous patch which introduced
>       kvm_mmu_prepare_zap_obsolete_page(), instead, we put the comments
>       around hash list walker to warn the user that the zapped-obsolete
>       page still live on hash list.
> 
>   5): add the note into the changelog of "zap pages in batch" patch to explain
>       the batch number is the speculative value based on Takuya's comments.
> 
> V7:
>   1): separate some optimization into two patches which do not reuse
>       the obsolete pages and collapse tlb flushes, suggested by Marcelo.
> 
>   2): make the patch based on Gleb's diff change which reduce
>       KVM_REQ_MMU_RELOAD when root page is being zapped.
> 
>   3): remove calling kvm_mmu_zap_page when patching hypercall, investigated
>       by Gleb.
> 
>   4): drop the patch which deleted page from hash list at the "prepare"
>       time since it can break the walk based on hash list.
> 
>   5): rename kvm_mmu_invalidate_all_pages to kvm_mmu_invalidate_zap_all_pages.
> 
>   6): introduce kvm_mmu_prepare_zap_obsolete_page which is used to zap obsolete
>       page to collapse tlb flushes.
> 
> V6:
>   1): reversely walk active_list to skip the new created pages based
>       on the comments from Gleb and Paolo.
> 
>   2): completely replace kvm_mmu_zap_all by kvm_mmu_invalidate_all_pages
>       based on Gleb's comments.
> 
>   3): improve the parameters of kvm_mmu_invalidate_all_pages based on
>       Gleb's comments.
>  
>   4): rename kvm_mmu_invalidate_memslot_pages to kvm_mmu_invalidate_all_pages
>   5): rename zap_invalid_pages to kvm_zap_obsolete_pages
> 
> 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.
> 
> Gleb Natapov (1):
>   KVM: MMU: reduce KVM_REQ_MMU_RELOAD when root page is zapped
> 
> Xiao Guangrong (10):
>   KVM: x86: drop calling kvm_mmu_zap_all in emulator_fix_hypercall
>   KVM: MMU: drop unnecessary kvm_reload_remote_mmus
>   KVM: MMU: fast invalidate all pages
>   KVM: x86: use the fast way to invalidate all pages
>   KVM: MMU: show mmu_valid_gen in shadow page related tracepoints
>   KVM: MMU: add tracepoint for kvm_mmu_invalidate_all_pages
>   KVM: MMU: do not reuse the obsolete page
>   KVM: MMU: zap pages in batch
>   KVM: MMU: collapse TLB flushes when zap all pages
>   KVM: MMU: reclaim the zapped-obsolete page first
> 
>  arch/x86/include/asm/kvm_host.h |    4 +
>  arch/x86/kvm/mmu.c              |  128 ++++++++++++++++++++++++++++++++++++---
>  arch/x86/kvm/mmu.h              |    1 +
>  arch/x86/kvm/mmutrace.h         |   42 ++++++++++---
>  arch/x86/kvm/x86.c              |   17 +----
>  5 files changed, 161 insertions(+), 31 deletions(-)
> 
> -- 
> 1.7.7.6

--
			Gleb.
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