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

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Date:	Fri, 3 Apr 2015 15:44:26 +0800
From:	Wanpeng Li <wanpeng.li@...ux.intel.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Paolo Bonzini <pbonzini@...hat.com>,
	Wanpeng Li <wanpeng.li@...ux.intel.com>
Subject: Re: [PATCH v2] kvm: mmu: lazy collapse small sptes into large sptes

On Fri, Apr 03, 2015 at 03:27:51PM +0800, Xiao Guangrong wrote:
>
>
>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. :)

I'm also not good at English, anyway, update it in v3. :)

>
>>+		 */
>>+		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...

Fix it in v3.

>
>Otherwise, it looks good to me.
>
>Reviewed-by: Xiao Guangrong <guangrong.xiao@...ux.intel.com>

Thanks for your review. :)

Regards,
Wanpeng Li 

>
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