Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > linux.kernel > #1581708 > unrolled thread

[PATCH 2/3] KVM: use separate generations for each address space

Back to article view | Back to linux.kernel

This discussion starts older than the indexed window; earlier articles aren't shown. The article labeled Started by below is the oldest one visible, not the original post.

Contents

  [PATCH 2/3] KVM: use separate generations for each address space Paolo Bonzini <pbonzini@redhat.com> - 2017-02-15 23:10 +0100
    Re: [PATCH 2/3] KVM: use separate generations for each address space Radim Krčmář <rkrcmar@redhat.com> - 2017-02-16 18:10 +0100
    Re: [PATCH 2/3] KVM: use separate generations for each address space Bandan Das <bsd@redhat.com> - 2017-02-17 01:30 +0100
      Re: [PATCH 2/3] KVM: use separate generations for each address  space Paolo Bonzini <pbonzini@redhat.com> - 2017-02-17 09:40 +0100

#1581708 — [PATCH 2/3] KVM: use separate generations for each address space

This will make it easier to support multiple address spaces in
kvm_gfn_to_hva_cache_init.  Instead of having to check the address
space id, we can keep on checking just the generation number.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 virt/kvm/kvm_main.c | 24 ++++++++++++++++--------
 1 file changed, 16 insertions(+), 8 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index e21bac7ed5d3..a83c186cefc1 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -506,11 +506,6 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
 	if (!slots)
 		return NULL;
 
-	/*
-	 * Init kvm generation close to the maximum to easily test the
-	 * code of handling generation number wrap-around.
-	 */
-	slots->generation = -150;
 	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
 		slots->id_to_index[i] = slots->memslots[i].id = i;
 
@@ -641,9 +636,16 @@ static struct kvm *kvm_create_vm(unsigned long type)
 
 	r = -ENOMEM;
 	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
-		kvm->memslots[i] = kvm_alloc_memslots();
-		if (!kvm->memslots[i])
+		struct kvm_memslots *slots = kvm_alloc_memslots();
+		if (!slots)
 			goto out_err_no_srcu;
+		/*
+		 * Generations must be different for each address space.
+		 * Init kvm generation close to the maximum to easily test the
+		 * code of handling generation number wrap-around.
+		 */
+		slots->generation = i * 2 - 150;
+		rcu_assign_pointer(kvm->memslots[i], slots);
 	}
 
 	if (init_srcu_struct(&kvm->srcu))
@@ -870,8 +872,14 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
 	 * Increment the new memslot generation a second time. This prevents
 	 * vm exits that race with memslot updates from caching a memslot
 	 * generation that will (potentially) be valid forever.
+	 *
+	 * Generations must be unique even across address spaces.  We do not need
+	 * a global counter for that, instead the generation space is evenly split
+	 * across address spaces.  For example, with two address spaces, address
+	 * space 0 will use generations 0, 4, 8, ... while * address space 1 will
+	 * use generations 2, 6, 10, 14, ...
 	 */
-	slots->generation++;
+	slots->generation += KVM_ADDRESS_SPACE_NUM * 2 - 1;
 
 	kvm_arch_memslots_updated(kvm, slots);
 
-- 
1.8.3.1

[toc] | [next] | [standalone]

#1582713

2017-02-15 23:00+0100, Paolo Bonzini:
> This will make it easier to support multiple address spaces in
> kvm_gfn_to_hva_cache_init.  Instead of having to check the address
> space id, we can keep on checking just the generation number.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Radim Krčmař <rkrcmar@redhat.com>

>  virt/kvm/kvm_main.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
> 
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e21bac7ed5d3..a83c186cefc1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -506,11 +506,6 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
>  	if (!slots)
>  		return NULL;
>  
> -	/*
> -	 * Init kvm generation close to the maximum to easily test the
> -	 * code of handling generation number wrap-around.
> -	 */
> -	slots->generation = -150;
>  	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
>  		slots->id_to_index[i] = slots->memslots[i].id = i;
>  
> @@ -641,9 +636,16 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  
>  	r = -ENOMEM;
>  	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> -		kvm->memslots[i] = kvm_alloc_memslots();
> -		if (!kvm->memslots[i])
> +		struct kvm_memslots *slots = kvm_alloc_memslots();
> +		if (!slots)
>  			goto out_err_no_srcu;
> +		/*
> +		 * Generations must be different for each address space.
> +		 * Init kvm generation close to the maximum to easily test the
> +		 * code of handling generation number wrap-around.
> +		 */
> +		slots->generation = i * 2 - 150;
> +		rcu_assign_pointer(kvm->memslots[i], slots);
>  	}
>  
>  	if (init_srcu_struct(&kvm->srcu))
> @@ -870,8 +872,14 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
>  	 * Increment the new memslot generation a second time. This prevents
>  	 * vm exits that race with memslot updates from caching a memslot
>  	 * generation that will (potentially) be valid forever.
> +	 *
> +	 * Generations must be unique even across address spaces.  We do not need
> +	 * a global counter for that, instead the generation space is evenly split
> +	 * across address spaces.  For example, with two address spaces, address
> +	 * space 0 will use generations 0, 4, 8, ... while * address space 1 will
> +	 * use generations 2, 6, 10, 14, ...
>  	 */
> -	slots->generation++;
> +	slots->generation += KVM_ADDRESS_SPACE_NUM * 2 - 1;
>  
>  	kvm_arch_memslots_updated(kvm, slots);
>  
> -- 
> 1.8.3.1
> 
> 

[toc] | [prev] | [next] | [standalone]

#1582996

Paolo Bonzini <pbonzini@redhat.com> writes:

> This will make it easier to support multiple address spaces in
> kvm_gfn_to_hva_cache_init.  Instead of having to check the address
> space id, we can keep on checking just the generation number.
>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  virt/kvm/kvm_main.c | 24 ++++++++++++++++--------
>  1 file changed, 16 insertions(+), 8 deletions(-)
>
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index e21bac7ed5d3..a83c186cefc1 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -506,11 +506,6 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
>  	if (!slots)
>  		return NULL;
>  
> -	/*
> -	 * Init kvm generation close to the maximum to easily test the
> -	 * code of handling generation number wrap-around.
> -	 */
> -	slots->generation = -150;
>  	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
>  		slots->id_to_index[i] = slots->memslots[i].id = i;
>  
> @@ -641,9 +636,16 @@ static struct kvm *kvm_create_vm(unsigned long type)
>  
>  	r = -ENOMEM;
>  	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> -		kvm->memslots[i] = kvm_alloc_memslots();
> -		if (!kvm->memslots[i])
> +		struct kvm_memslots *slots = kvm_alloc_memslots();
> +		if (!slots)
>  			goto out_err_no_srcu;
> +		/*
> +		 * Generations must be different for each address space.
> +		 * Init kvm generation close to the maximum to easily test the
> +		 * code of handling generation number wrap-around.
> +		 */
> +		slots->generation = i * 2 - 150;
> +		rcu_assign_pointer(kvm->memslots[i], slots);
>  	}

I can't seem to understand why rcu_assign_pointer wasn't used before.
kvm->memslots[i] was a rcu protected pointer even before this change,
right ?

>  	if (init_srcu_struct(&kvm->srcu))
> @@ -870,8 +872,14 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
>  	 * Increment the new memslot generation a second time. This prevents
>  	 * vm exits that race with memslot updates from caching a memslot
>  	 * generation that will (potentially) be valid forever.
> +	 *
> +	 * Generations must be unique even across address spaces.  We do not need
> +	 * a global counter for that, instead the generation space is evenly split
> +	 * across address spaces.  For example, with two address spaces, address
> +	 * space 0 will use generations 0, 4, 8, ... while * address space 1 will
> +	 * use generations 2, 6, 10, 14, ...
>  	 */
> -	slots->generation++;
> +	slots->generation += KVM_ADDRESS_SPACE_NUM * 2 - 1;
>  
>  	kvm_arch_memslots_updated(kvm, slots);

[toc] | [prev] | [next] | [standalone]

#1583198 — Re: [PATCH 2/3] KVM: use separate generations for each address space

> > +		/*
> > +		 * Generations must be different for each address space.
> > +		 * Init kvm generation close to the maximum to easily test the
> > +		 * code of handling generation number wrap-around.
> > +		 */
> > +		slots->generation = i * 2 - 150;
> > +		rcu_assign_pointer(kvm->memslots[i], slots);
> >  	}
> 
> I can't seem to understand why rcu_assign_pointer wasn't used before.
> kvm->memslots[i] was a rcu protected pointer even before this change,
> right ?

Actually, a better match is RCU_INIT_POINTER.  Here there is no concurrent
reader because we're just initializing the struct kvm.  There is something
else providing synchronization between this writer and the "first" RCU
read-side.  It could be signaling a condition variable, creating a thread,
or releasing a mutex; all three of them have release semantics, which
means they imply a smp_wmb just like rcu_assign_pointer does.

Paolo


> >  	if (init_srcu_struct(&kvm->srcu))
> > @@ -870,8 +872,14 @@ static struct kvm_memslots
> > *install_new_memslots(struct kvm *kvm,
> >  	 * Increment the new memslot generation a second time. This prevents
> >  	 * vm exits that race with memslot updates from caching a memslot
> >  	 * generation that will (potentially) be valid forever.
> > +	 *
> > +	 * Generations must be unique even across address spaces.  We do not need
> > +	 * a global counter for that, instead the generation space is evenly
> > split
> > +	 * across address spaces.  For example, with two address spaces, address
> > +	 * space 0 will use generations 0, 4, 8, ... while * address space 1 will
> > +	 * use generations 2, 6, 10, 14, ...
> >  	 */
> > -	slots->generation++;
> > +	slots->generation += KVM_ADDRESS_SPACE_NUM * 2 - 1;
> >  
> >  	kvm_arch_memslots_updated(kvm, slots);
> 

[toc] | [prev] | [standalone]

Back to top | Article view | linux.kernel

csiph-web