Re: [PATCH v3 13/15] KVM: MMU: locklessly write-protect the page

[Gleb Natapov]

On Wed, Oct 23, 2013 at 09:29:31PM +0800, Xiao Guangrong wrote:
> Currently, when mark memslot dirty logged or get dirty page, we need to
> write-protect large guest memory, it is the heavy work, especially, we
> need to hold mmu-lock which is also required by vcpu to fix its page table
> fault and mmu-notifier when host page is being changed. In the extreme
> cpu / memory used guest, it becomes a scalability issue
> 
> This patch introduces a way to locklessly write-protect guest memory
> 
> Now, lockless rmap walk, lockless shadow page table access and lockless
> spte wirte-protection are ready, it is the time to implements page
> write-protection out of mmu-lock
> 
> Signed-off-by: Xiao Guangrong <xiaoguangrong@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm_host.h |  4 ---
>  arch/x86/kvm/mmu.c              | 59 ++++++++++++++++++++++++++++++-----------
>  arch/x86/kvm/mmu.h              |  6 +++++
>  arch/x86/kvm/x86.c              | 11 ++++----
>  4 files changed, 55 insertions(+), 25 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index df9ae10..cdb6f29 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -793,10 +793,6 @@ void kvm_mmu_set_mask_ptes(u64 user_mask, u64 accessed_mask,
>  		u64 dirty_mask, u64 nx_mask, u64 x_mask);
>  
>  void kvm_mmu_reset_context(struct kvm_vcpu *vcpu);
> -void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot);
> -void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> -				     struct kvm_memory_slot *slot,
> -				     gfn_t gfn_offset, unsigned long mask);
>  void kvm_mmu_zap_all(struct kvm *kvm);
>  void kvm_mmu_invalidate_mmio_sptes(struct kvm *kvm);
>  unsigned int kvm_mmu_calculate_mmu_pages(struct kvm *kvm);
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 8b96d96..d82bbec 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -1386,8 +1386,37 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
>  	return flush;
>  }
>  
> -/**
> - * kvm_mmu_write_protect_pt_masked - write protect selected PT level pages
> +static void __rmap_write_protect_lockless(u64 *sptep)
> +{
> +	u64 spte;
> +
> +retry:
> +	/*
> +	 * Note we may partly read the sptep on 32bit host, however, we
> +	 * allow this case because:
> +	 * - we do not access the page got from the sptep.
> +	 * - cmpxchg64 can detect that case and avoid setting a wrong value
> +	 *   to the sptep.
> +	 */
> +	spte = *rcu_dereference(sptep);
> +	if (unlikely(!is_last_spte(spte) || !is_writable_pte(spte)))
is_last_spte gets two parameters.

> +		return;
> +
> +	if (likely(cmpxchg64(sptep, spte, spte & ~PT_WRITABLE_MASK) == spte))
> +		return;
> +
> +	goto retry;
> +}
> +
> +static void rmap_write_protect_lockless(unsigned long *rmapp)
> +{
> +	pte_list_walk_lockless(rmapp, __rmap_write_protect_lockless);
> +}
> +
> +/*
> + * kvm_mmu_write_protect_pt_masked_lockless - write protect selected PT level
> + * pages out of mmu-lock.
> + *
>   * @kvm: kvm instance
>   * @slot: slot to protect
>   * @gfn_offset: start of the BITS_PER_LONG pages we care about
> @@ -1396,16 +1425,17 @@ static bool __rmap_write_protect(struct kvm *kvm, unsigned long *rmapp,
>   * Used when we do not need to care about huge page mappings: e.g. during dirty
>   * logging we do not have any such mappings.
>   */
> -void kvm_mmu_write_protect_pt_masked(struct kvm *kvm,
> -				     struct kvm_memory_slot *slot,
> -				     gfn_t gfn_offset, unsigned long mask)
> +void
> +kvm_mmu_write_protect_pt_masked_lockless(struct kvm *kvm,
> +					 struct kvm_memory_slot *slot,
> +					 gfn_t gfn_offset, unsigned long mask)
>  {
>  	unsigned long *rmapp;
>  
>  	while (mask) {
>  		rmapp = __gfn_to_rmap(slot->base_gfn + gfn_offset + __ffs(mask),
>  				      PT_PAGE_TABLE_LEVEL, slot);
> -		__rmap_write_protect(kvm, rmapp, false);
> +		rmap_write_protect_lockless(rmapp);
>  
>  		/* clear the first set bit */
>  		mask &= mask - 1;
> @@ -4477,7 +4507,7 @@ void kvm_mmu_setup(struct kvm_vcpu *vcpu)
>  	init_kvm_mmu(vcpu);
>  }
>  
> -void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
> +void kvm_mmu_slot_remove_write_access_lockless(struct kvm *kvm, int slot)
>  {
>  	struct kvm_memory_slot *memslot;
>  	gfn_t last_gfn;
> @@ -4486,8 +4516,7 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
>  	memslot = id_to_memslot(kvm->memslots, slot);
>  	last_gfn = memslot->base_gfn + memslot->npages - 1;
>  
> -	spin_lock(&kvm->mmu_lock);
> -
> +	rcu_read_lock();
>  	for (i = PT_PAGE_TABLE_LEVEL;
>  	     i < PT_PAGE_TABLE_LEVEL + KVM_NR_PAGE_SIZES; ++i) {
>  		unsigned long *rmapp;
> @@ -4497,15 +4526,15 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
>  		last_index = gfn_to_index(last_gfn, memslot->base_gfn, i);
>  
>  		for (index = 0; index <= last_index; ++index, ++rmapp) {
> -			if (*rmapp)
> -				__rmap_write_protect(kvm, rmapp, false);
> +			rmap_write_protect_lockless(rmapp);
>  
> -			if (need_resched() || spin_needbreak(&kvm->mmu_lock))
> -				cond_resched_lock(&kvm->mmu_lock);
> +			if (need_resched()) {
> +				rcu_read_lock();
> +				rcu_read_unlock();
> +			}
>  		}
>  	}
> -
> -	spin_unlock(&kvm->mmu_lock);
> +	rcu_read_unlock();
>  
>  	/*
>  	 * We can flush all the TLBs out of the mmu lock without TLB
> diff --git a/arch/x86/kvm/mmu.h b/arch/x86/kvm/mmu.h
> index 2926152..33f313b 100644
> --- a/arch/x86/kvm/mmu.h
> +++ b/arch/x86/kvm/mmu.h
> @@ -117,4 +117,10 @@ static inline bool permission_fault(struct kvm_mmu *mmu, unsigned pte_access,
>  }
>  
>  void kvm_mmu_invalidate_zap_all_pages(struct kvm *kvm);
> +
> +void kvm_mmu_slot_remove_write_access_lockless(struct kvm *kvm, int slot);
> +void
> +kvm_mmu_write_protect_pt_masked_lockless(struct kvm *kvm,
> +					 struct kvm_memory_slot *slot,
> +					 gfn_t gfn_offset, unsigned long mask);
>  #endif
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 4ac3a27..c6233e1 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3554,8 +3554,7 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
>  	dirty_bitmap_buffer = dirty_bitmap + n / sizeof(long);
>  	memset(dirty_bitmap_buffer, 0, n);
>  
> -	spin_lock(&kvm->mmu_lock);
> -
> +	rcu_read_lock();
>  	for (i = 0; i < n / sizeof(long); i++) {
>  		unsigned long mask;
>  		gfn_t offset;
> @@ -3579,10 +3578,10 @@ int kvm_vm_ioctl_get_dirty_log(struct kvm *kvm, struct kvm_dirty_log *log)
>  		dirty_bitmap_buffer[i] = mask;
>  
>  		offset = i * BITS_PER_LONG;
> -		kvm_mmu_write_protect_pt_masked(kvm, memslot, offset, mask);
> +		kvm_mmu_write_protect_pt_masked_lockless(kvm, memslot,
> +							 offset, mask);
>  	}
> -
> -	spin_unlock(&kvm->mmu_lock);
> +	rcu_read_unlock();
>  
>  	/*
>  	 * All the TLBs can be flushed out of mmu lock, see the comments in
> @@ -7246,7 +7245,7 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>  	 * See the comments in fast_page_fault().
>  	 */
>  	if ((change != KVM_MR_DELETE) && (mem->flags & KVM_MEM_LOG_DIRTY_PAGES))
> -		kvm_mmu_slot_remove_write_access(kvm, mem->slot);
> +		kvm_mmu_slot_remove_write_access_lockless(kvm, mem->slot);
>  }
>  
>  void kvm_arch_flush_shadow_all(struct kvm *kvm)
> -- 
> 1.8.1.4

--
			Gleb.
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