linux-kernel - Re: [PATCH v2] kvm: mmu: lazy collapse small sptes into large sptes

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Date:	Fri, 03 Apr 2015 15:27:51 +0800
From:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
CC:	Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [PATCH v2] kvm: mmu: lazy collapse small sptes into large sptes



On 04/03/2015 02:09 PM, Wanpeng Li wrote:
> There are two scenarios for the requirement of collapsing small sptes
> into large sptes.
> - dirty logging tracks sptes in 4k granularity, so large sptes are split,
>    the large sptes will be reallocated in the destination machine and the
>    guest in the source machine will be destroyed when live migration successfully.
>    However, the guest in the source machine will continue to run if live migration
>    fail due to some reasons, the sptes still keep small which lead to bad
>    performance.
> - our customers write tools to track the dirty speed of guests by EPT D bit/PML
>    in order to determine the most appropriate one to be live migrated, however
>    sptes will still keep small after tracking dirty speed.
>
> This patch introduce lazy collapse small sptes into large sptes, the memory region
> will be scanned on the ioctl context when dirty log is stopped, the ones which can
> be collapsed into large pages will be dropped during the scan, it depends the on
> later #PF to reallocate all large sptes.
>
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
> v1 -> v2:
>   * use 'bool' instead of 'int'
>   * add more comments
>   * fix can not get the next spte after drop the current spte
>
>   arch/x86/include/asm/kvm_host.h |  2 ++
>   arch/x86/kvm/mmu.c              | 71 +++++++++++++++++++++++++++++++++++++++++
>   arch/x86/kvm/x86.c              | 19 +++++++++++
>   3 files changed, 92 insertions(+)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 30b28dc..91b5bdb 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -854,6 +854,8 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
>   void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
>   void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
>   				      struct kvm_memory_slot *memslot);
> +void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
> +					struct kvm_memory_slot *memslot);
>   void kvm_mmu_slot_leaf_clear_dirty(struct kvm *kvm,
>   				   struct kvm_memory_slot *memslot);
>   void kvm_mmu_slot_largepage_remove_write_access(struct kvm *kvm,
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index cee7592..df3f2e3 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4465,6 +4465,77 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm,
>   		kvm_flush_remote_tlbs(kvm);
>   }
>
> +static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> +		unsigned long *rmapp)
> +{
> +	u64 *sptep;
> +	struct rmap_iterator iter;
> +	int need_tlb_flush = 0;
> +	pfn_t pfn;
> +	struct kvm_mmu_page *sp;
> +
> +	while ((sptep = rmap_get_first(*rmapp, &iter))) {
> +		BUG_ON(!(*sptep & PT_PRESENT_MASK));
> +
> +		sp = page_header(__pa(sptep));
> +		pfn = spte_to_pfn(*sptep);
> +
> +		/*
> +		 * Let support EPT only now, an efficient way need to be figure
> +		 * out to let these code be aware what mapping level used in
> +		 * guest.

This English seems strange... but i am not good at it. :)

> +		 */
> +		if (sp->role.direct &&
> +			!kvm_is_reserved_pfn(pfn) &&
> +			PageTransCompound(pfn_to_page(pfn))) {
> +			drop_spte(kvm, sptep);
> +			need_tlb_flush = 1;
> +		}

If the conditions are not comfortable, it does loop forever...

Otherwise, it looks good to me.

Reviewed-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>


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