linux-kernel - Re: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes

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Date:	Thu, 13 Jun 2013 21:08:21 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
Cc:	gleb@...hat.com, avi.kivity@...il.com, pbonzini@...hat.com,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 0/6] KVM: MMU: fast invalidate all mmio sptes

On Fri, Jun 07, 2013 at 04:51:22PM +0800, Xiao Guangrong wrote:
> Changelog:
> V3:
>   All of these changes are from Gleb's review:
>   1) rename RET_MMIO_PF_EMU to RET_MMIO_PF_EMULATE.
>   2) smartly adjust kvm generation number in kvm_current_mmio_generatio()
>      to avoid kvm_memslots->generation overflow.
> 
> 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 invalidate all mmio sptes - it need not walk any shadow pages and hold
> any locks.

Hi Xiao,

- Where is the generation number increased?
- Should use spinlock breakable code in kvm_mmu_zap_mmio_sptes()
(picture guest with 512GB of RAM, even walking all those pages is
expensive) (ah, patch to remove kvm_mmu_zap_mmio_sptes does that).
- Is -13 enough to test wraparound? Its highly likely the guest has 
not began executing by the time 13 kvm_set_memory_calls are made
(so no sptes around). Perhaps -2000 is more sensible (should confirm
though).
- Why remove "if (change == KVM_MR_CREATE) || (change
==  KVM_MR_MOVE)" from kvm_arch_commit_memory_region?
Its instructive.

Otherwise looks good.

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