Re: [PATCH v5 18/19] KVM: Dynamically size memslot array based on number of used slots

[Peter Xu]

On Tue, Jan 21, 2020 at 02:31:56PM -0800, Sean Christopherson wrote:
> Now that the memslot logic doesn't assume memslots are always non-NULL,
> dynamically size the array of memslots instead of unconditionally
> allocating memory for the maximum number of memslots.
> 
> Note, because a to-be-deleted memslot must first be invalidated, the
> array size cannot be immediately reduced when deleting a memslot.
> However, consecutive deletions will realize the memory savings, i.e.
> a second deletion will trim the entry.
> 
> Tested-by: Christoffer Dall <christoffer.dall@xxxxxxx>
> Tested-by: Marc Zyngier <maz@xxxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
>  include/linux/kvm_host.h |  2 +-
>  virt/kvm/kvm_main.c      | 31 ++++++++++++++++++++++++++++---
>  2 files changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 60ddfdb69378..8bb6fb127387 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -431,11 +431,11 @@ static inline int kvm_arch_vcpu_memslots_id(struct kvm_vcpu *vcpu)
>   */
>  struct kvm_memslots {
>  	u64 generation;
> -	struct kvm_memory_slot memslots[KVM_MEM_SLOTS_NUM];
>  	/* The mapping table from slot id to the index in memslots[]. */
>  	short id_to_index[KVM_MEM_SLOTS_NUM];
>  	atomic_t lru_slot;
>  	int used_slots;
> +	struct kvm_memory_slot memslots[];

This patch is tested so I believe this works, however normally I need
to do similar thing with [0] otherwise gcc might complaint.  Is there
any trick behind to make this work?  Or is that because of different
gcc versions?

>  };
>  
>  struct kvm {
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 9b614cf2ca20..ed392ce64e59 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -565,7 +565,7 @@ static struct kvm_memslots *kvm_alloc_memslots(void)
>  		return NULL;
>  
>  	for (i = 0; i < KVM_MEM_SLOTS_NUM; i++)
> -		slots->id_to_index[i] = slots->memslots[i].id = -1;
> +		slots->id_to_index[i] = -1;
>  
>  	return slots;
>  }
> @@ -1077,6 +1077,32 @@ static struct kvm_memslots *install_new_memslots(struct kvm *kvm,
>  	return old_memslots;
>  }
>  
> +/*
> + * Note, at a minimum, the current number of used slots must be allocated, even
> + * when deleting a memslot, as we need a complete duplicate of the memslots for
> + * use when invalidating a memslot prior to deleting/moving the memslot.
> + */
> +static struct kvm_memslots *kvm_dup_memslots(struct kvm_memslots *old,
> +					     enum kvm_mr_change change)
> +{
> +	struct kvm_memslots *slots;
> +	size_t old_size, new_size;
> +
> +	old_size = sizeof(struct kvm_memslots) +
> +		   (sizeof(struct kvm_memory_slot) * old->used_slots);
> +
> +	if (change == KVM_MR_CREATE)
> +		new_size = old_size + sizeof(struct kvm_memory_slot);
> +	else
> +		new_size = old_size;
> +
> +	slots = kvzalloc(new_size, GFP_KERNEL_ACCOUNT);
> +	if (likely(slots))
> +		memcpy(slots, old, old_size);

(Maybe directly copy into it?)

> +
> +	return slots;
> +}
> +
>  static int kvm_set_memslot(struct kvm *kvm,
>  			   const struct kvm_userspace_memory_region *mem,
>  			   struct kvm_memory_slot *old,
> @@ -1087,10 +1113,9 @@ static int kvm_set_memslot(struct kvm *kvm,
>  	struct kvm_memslots *slots;
>  	int r;
>  
> -	slots = kvzalloc(sizeof(struct kvm_memslots), GFP_KERNEL_ACCOUNT);
> +	slots = kvm_dup_memslots(__kvm_memslots(kvm, as_id), change);
>  	if (!slots)
>  		return -ENOMEM;
> -	memcpy(slots, __kvm_memslots(kvm, as_id), sizeof(struct kvm_memslots));
>  
>  	if (change == KVM_MR_DELETE || change == KVM_MR_MOVE) {
>  		/*
> -- 
> 2.24.1
> 

-- 
Peter Xu