linux-kernel - Re: [PATCH 2/3] KVM: use separate generations for each address space

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Date:   Thu, 16 Feb 2017 19:29:15 -0500
From:   Bandan Das <bsd@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: use separate generations for each address space

Paolo Bonzini <pbonzini@...hat.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@...hat.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);