Re: [PATCH v2 7/7] KVM: x86/mmu: Lazily allocate memslot rmaps

[Sean Christopherson <seanjc@google.com>]

On Thu, Apr 29, 2021, Ben Gardon wrote:
> If the TDP MMU is in use, wait to allocate the rmaps until the shadow
> MMU is actually used. (i.e. a nested VM is launched.) This saves memory
> equal to 0.2% of guest memory in cases where the TDP MMU is used and
> there are no nested guests involved.
> 
> Signed-off-by: Ben Gardon <bgardon@google.com>
> ---
>  arch/x86/include/asm/kvm_host.h | 11 +++++++
>  arch/x86/kvm/mmu/mmu.c          | 21 +++++++++++--
>  arch/x86/kvm/mmu/mmu_internal.h |  2 +-
>  arch/x86/kvm/x86.c              | 54 ++++++++++++++++++++++++++++++---
>  4 files changed, 80 insertions(+), 8 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 3900dcf2439e..b8633ed00a6a 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1124,6 +1124,15 @@ struct kvm_arch {
>  #endif /* CONFIG_X86_64 */
>  
>  	bool shadow_mmu_active;
> +
> +	/*
> +	 * If set, the rmap should be allocated for any newly created or
> +	 * modified memslots. If allocating rmaps lazily, this may be set
> +	 * before the rmaps are allocated for existing memslots, but
> +	 * shadow_mmu_active will not be set until after the rmaps are fully
> +	 * allocated.
> +	 */
> +	bool alloc_memslot_rmaps;

Maybe "need_rmaps" or "need_memslot_rmaps"?

>  };
>  
>  struct kvm_vm_stat {
> @@ -1855,4 +1864,6 @@ static inline int kvm_cpu_get_apicid(int mps_cpu)
>  
>  int kvm_cpu_dirty_log_size(void);
>  
> +int alloc_all_memslots_rmaps(struct kvm *kvm);
> +
>  #endif /* _ASM_X86_KVM_HOST_H */
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index e252af46f205..b2a6585bd978 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3125,9 +3125,17 @@ static int fast_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>  	return ret;
>  }
>  
> -void activate_shadow_mmu(struct kvm *kvm)
> +int activate_shadow_mmu(struct kvm *kvm)
>  {
> +	int r;
> +
> +	r = alloc_all_memslots_rmaps(kvm);
> +	if (r)
> +		return r;
> +
>  kvm->arch.shadow_mmu_active = true;

If shadow_mmu_active goes away, so does this helper.

> +
> +	return 0;
>  }
>  
>  static void mmu_free_root_page(struct kvm *kvm, hpa_t *root_hpa,
> @@ -3300,7 +3308,9 @@ static int mmu_alloc_shadow_roots(struct kvm_vcpu *vcpu)
>  		}
>  	}
>  
> -	activate_shadow_mmu(vcpu->kvm);
> +	r = activate_shadow_mmu(vcpu->kvm);
> +	if (r)
> +		return r;
>  
>  	write_lock(&vcpu->kvm->mmu_lock);
>  	r = make_mmu_pages_available(vcpu);
> @@ -5491,7 +5501,12 @@ void kvm_mmu_init_vm(struct kvm *kvm)
>  	struct kvm_page_track_notifier_node *node = &kvm->arch.mmu_sp_tracker;
>  
>  	if (!kvm_mmu_init_tdp_mmu(kvm))
> -		activate_shadow_mmu(kvm);
> +		/*
> +		 * No memslots can have been allocated at this point.
> +		 * activate_shadow_mmu won't actually need to allocate
> +		 * rmaps, so it cannot fail.
> +		 */
> +		WARN_ON(activate_shadow_mmu(kvm));

This is where I really don't like calling the full flow.  VM init is already
special, I don't see any harm in open coding the setting of the flag.  This also
provides a good place to document that the smp_store/load business is unnecessary
since there can't be users.

>  	node->track_write = kvm_mmu_pte_write;
>  	node->track_flush_slot = kvm_mmu_invalidate_zap_pages_in_memslot;
> -static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
> +int alloc_memslots_rmaps(struct kvm *kvm, struct kvm_memslots *slots)
> +{
> +	struct kvm_memory_slot *slot;
> +	int r = 0;
> +
> +	kvm_for_each_memslot(slot, slots) {
> +		r = alloc_memslot_rmap(kvm, slot, slot->npages);
> +		if (r)
> +			break;
> +	}
> +	return r;
> +}

Just open code this in the caller, it's literally one line of code and the
indentation isn't bad.

> +
> +int alloc_all_memslots_rmaps(struct kvm *kvm)
> +{
> +	struct kvm_memslots *slots;
> +	int r = 0;
> +	int i;
> +
> +	mutex_lock(&kvm->slots_arch_lock);
> +	kvm->arch.alloc_memslot_rmaps = true;
> +
> +	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
> +		slots = __kvm_memslots(kvm, i);
> +		r = alloc_memslots_rmaps(kvm, slots);
> +		if (r)

It'd be easier just to destroy the rmaps on failure and then do:

	if (kvm->arch.needs_memslots_rmaps)
		return;

	mutex_lock(&kvm->slots_arch_lock);

	for (i = 0; i < KVM_ADDRESS_SPACE_NUM; i++) {
		kvm_for_each_memslot(slot, __kvm_memslots(kvm, i)) {
			r = alloc_memslot_rmap(kvm, slot, slot->npages);
				break;
		}
	}

	if (!r)
		smp_store_release(kvm->arch.needs_memslots_rmaps, true);
	else
		kvm_free_rmaps(kvm);
	mutex_unlock(&kvm->slots_arch_lock);


and make alloc_memslot_rmap() a pure allocator (no checks on whether it should
actually do allocations), i.e. push the check to the memslot flow:

static int kvm_alloc_memslot_metadata(struct kvm *kvm,
				      struct kvm_memory_slot *slot,
				      unsigned long npages)
{
	int i;
	int r;

	/*
	 * Clear out the previous array pointers for the KVM_MR_MOVE case.  The
	 * old arrays will be freed by __kvm_set_memory_region() if installing
	 * the new memslot is successful.
	 */
	memset(&slot->arch, 0, sizeof(slot->arch));

	if (kvm->arch.needs_memslots_rmaps) {
		r = alloc_memslot_rmap(kvm, slot, npages);
		if (r)
			return r;
	}


With that, there's no need for the separate shadow_mmu_active flag, and you can
do s/activate_shadow_mmu/kvm_activate_rmaps or so.


> +			break;
> +	}
> +	mutex_unlock(&kvm->slots_arch_lock);
> +	return r;
> +}
> +
> +static int kvm_alloc_memslot_metadata(struct kvm *kvm,
> +				      struct kvm_memory_slot *slot,
>  				      unsigned long npages)
>  {
>  	int i;
> @@ -10881,7 +10927,7 @@ static int kvm_alloc_memslot_metadata(struct kvm_memory_slot *slot,
>  	 */
>  	memset(&slot->arch, 0, sizeof(slot->arch));
>  
> -	r = alloc_memslot_rmap(slot, npages);
> +	r = alloc_memslot_rmap(kvm, slot, npages);
>  	if (r)
>  		return r;
>  
> @@ -10954,7 +11000,7 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm,
>  				enum kvm_mr_change change)
>  {
>  	if (change == KVM_MR_CREATE || change == KVM_MR_MOVE)
> -		return kvm_alloc_memslot_metadata(memslot,
> +		return kvm_alloc_memslot_metadata(kvm, memslot,
>  						  mem->memory_size >> PAGE_SHIFT);
>  	return 0;
>  }
> -- 
> 2.31.1.527.g47e6f16901-goog
>