linux-kernel - Re: [PATCH v2 0/6] KVM: MMU: fast invalid all mmio sptes

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Date:	Mon, 15 Apr 2013 21:54:39 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v2 0/6] KVM: MMU: fast invalid all mmio sptes

On Mon, Apr 01, 2013 at 05:56:43PM +0800, Xiao Guangrong wrote:
> Changelog in v2:
>   - rename kvm_mmu_invalid_mmio_spte to kvm_mmu_invalid_mmio_sptes
>   - use kvm->memslots->generation as kvm global generation-number
>   - fix comment and codestyle
>   - init kvm generation close to mmio wrap-around value
>   - keep kvm_mmu_zap_mmio_sptes
> 
> The current way is holding hot mmu-lock and walking all shadow pages, this
> is not scale. This patchset tries to introduce a very simple and scale way
> to fast invalid all mmio sptes - it need not walk any shadow pages and hold
> any locks.
> 
> The idea is simple:
> KVM maintains a global mmio invalid generation-number which is stored in
> kvm->memslots.generation and every mmio spte stores the current global
> generation-number into his available bits when it is created
> 
> When KVM need zap all mmio sptes, it just simply increase the global
> generation-number. When guests do mmio access, KVM intercepts a MMIO #PF
> then it walks the shadow page table and get the mmio spte. If the
> generation-number on the spte does not equal the global generation-number,
> it will go to the normal #PF handler to update the mmio spte
> 
> Since 19 bits are used to store generation-number on mmio spte, we zap all
> mmio sptes when the number is round

Hi Xiao,

Is it still necessary with generation numbers at 'struct shadow_page'
level (which covers the slot deletion case).

That is, once kvm_mmu_zap_all is fixed to increase generation count and
nuke roots, can't that be used instead with similar effectiveness for
SLOT_CREATE/SLOT_MOVE cases?

> Xiao Guangrong (6):
>   KVM: MMU: retain more available bits on mmio spte
>   KVM: MMU: store generation-number into mmio spte
>   KVM: MMU: make return value of mmio page fault handler more readable
>   KVM: MMU: fast invalid all mmio sptes
>   KVM: MMU: add tracepoint for check_mmio_spte
>   KVM: MMU: init kvm generation close to mmio wrap-around value
> 
>  arch/x86/include/asm/kvm_host.h |    3 +-
>  arch/x86/kvm/mmu.c              |  134 +++++++++++++++++++++++++++++++--------
>  arch/x86/kvm/mmu.h              |   17 +++++
>  arch/x86/kvm/mmutrace.h         |   34 +++++++++-
>  arch/x86/kvm/paging_tmpl.h      |   10 ++-
>  arch/x86/kvm/vmx.c              |   12 +++-
>  arch/x86/kvm/x86.c              |   11 +++-
>  virt/kvm/kvm_main.c             |    6 ++
>  8 files changed, 186 insertions(+), 41 deletions(-)
> 
> -- 
> 1.7.7.6
> 
> --
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