Re: [PATCH A] KVM: Simplify kvm->tlbs_dirty handling

[Paolo Bonzini <pbonzini@redhat.com>]

Il 18/02/2014 09:22, Takuya Yoshikawa ha scritto:
> When this was introduced, kvm_flush_remote_tlbs() could be called
> without holding mmu_lock.  It is now acknowledged that the function
> must be called before releasing mmu_lock, and all callers have already
> been changed to do so.
> 
> There is no need to use smp_mb() and cmpxchg() any more.
> 
> Signed-off-by: Takuya Yoshikawa <yoshikawa_takuya_b1@lab.ntt.co.jp>

I prefer this patch, and in fact we can make it even simpler:

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index ed1cc89..9816b68 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -401,7 +401,9 @@ struct kvm {
 	unsigned long mmu_notifier_seq;
 	long mmu_notifier_count;
 #endif
+	/* Protected by mmu_lock */
 	bool tlbs_dirty;
+
 	struct list_head devices;
 };
 
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 51744da..f5668a4 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -188,12 +188,6 @@ void kvm_flush_remote_tlbs(struct kvm *kvm)
 {
 	if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
 		++kvm->stat.remote_tlb_flush;
-	/*
-	 * tlbs_dirty is used only for optimizing x86's shadow paging code with
-	 * mmu notifiers in mind, see the note on sync_page().  Since it is
-	 * always protected with mmu_lock there, should kvm_flush_remote_tlbs()
-	 * be called before releasing mmu_lock, this is safe.
-	 */
 	kvm->tlbs_dirty = false;
 }
 EXPORT_SYMBOL_GPL(kvm_flush_remote_tlbs);


What do you think?

Paolo
> ---
>  arch/x86/kvm/paging_tmpl.h |    7 ++++---
>  include/linux/kvm_host.h   |    2 +-
>  virt/kvm/kvm_main.c        |   11 +++++++----
>  3 files changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/kvm/paging_tmpl.h b/arch/x86/kvm/paging_tmpl.h
> index cba218a..b1e6c1b 100644
> --- a/arch/x86/kvm/paging_tmpl.h
> +++ b/arch/x86/kvm/paging_tmpl.h
> @@ -913,7 +913,8 @@ static gpa_t FNAME(gva_to_gpa_nested)(struct kvm_vcpu *vcpu, gva_t vaddr,
>   *   and kvm_mmu_notifier_invalidate_range_start detect the mapping page isn't
>   *   used by guest then tlbs are not flushed, so guest is allowed to access the
>   *   freed pages.
> - *   And we increase kvm->tlbs_dirty to delay tlbs flush in this case.
> + *   We set tlbs_dirty to let the notifier know this change and delay the flush
> + *   until such a case actually happens.
>   */
>  static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  {
> @@ -942,7 +943,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  			return -EINVAL;
>  
>  		if (FNAME(prefetch_invalid_gpte)(vcpu, sp, &sp->spt[i], gpte)) {
> -			vcpu->kvm->tlbs_dirty++;
> +			vcpu->kvm->tlbs_dirty = true;
>  			continue;
>  		}
>  
> @@ -957,7 +958,7 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp)
>  
>  		if (gfn != sp->gfns[i]) {
>  			drop_spte(vcpu->kvm, &sp->spt[i]);
> -			vcpu->kvm->tlbs_dirty++;
> +			vcpu->kvm->tlbs_dirty = true;
>  			continue;
>  		}
>  
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index f5937b8..ed1cc89 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -401,7 +401,7 @@ struct kvm {
>  	unsigned long mmu_notifier_seq;
>  	long mmu_notifier_count;
>  #endif
> -	long tlbs_dirty;
> +	bool tlbs_dirty;
>  	struct list_head devices;
>  };
>  
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index a9e999a..51744da 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -186,12 +186,15 @@ static bool make_all_cpus_request(struct kvm *kvm, unsigned int req)
>  
>  void kvm_flush_remote_tlbs(struct kvm *kvm)
>  {
> -	long dirty_count = kvm->tlbs_dirty;
> -
> -	smp_mb();
>  	if (make_all_cpus_request(kvm, KVM_REQ_TLB_FLUSH))
>  		++kvm->stat.remote_tlb_flush;
> -	cmpxchg(&kvm->tlbs_dirty, dirty_count, 0);
> +	/*
> +	 * tlbs_dirty is used only for optimizing x86's shadow paging code with
> +	 * mmu notifiers in mind, see the note on sync_page().  Since it is
> +	 * always protected with mmu_lock there, should kvm_flush_remote_tlbs()
> +	 * be called before releasing mmu_lock, this is safe.
> +	 */
> +	kvm->tlbs_dirty = false;
>  }
>  EXPORT_SYMBOL_GPL(kvm_flush_remote_tlbs);
>  
>