linux-kernel - Re: [PATCH v2 1/3] KVM: x86/mmu: Dynamically allocate shadow MMU's hashed page list

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Message-ID: <20250416155349.GA701848.vipinsh@google.com>
Date: Wed, 16 Apr 2025 08:53:49 -0700
From: Vipin Sharma <vipinsh@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: x86/mmu: Dynamically allocate shadow MMU's
 hashed page list

On 2025-04-01 08:57:12, Sean Christopherson wrote:
> Dynamically allocate the (massive) array of hashed lists used to track
> shadow pages, as the array itself is 32KiB, i.e. is an order-3 allocation
> all on its own, and is *exactly* an order-3 allocation.  Dynamically
> allocating the array will allow allocating "struct kvm" using regular
> kmalloc(), and will also allow deferring allocation of the array until
> it's actually needed, i.e. until the first shadow root is allocated.
> 
> Cc: Vipin Sharma <vipinsh@...gle.com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/include/asm/kvm_host.h |  4 ++--
>  arch/x86/kvm/mmu/mmu.c          | 23 ++++++++++++++++++++++-
>  arch/x86/kvm/x86.c              |  5 ++++-
>  3 files changed, 28 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index a884ab544335..e523d7d8a107 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1333,7 +1333,7 @@ struct kvm_arch {
>  	bool has_private_mem;
>  	bool has_protected_state;
>  	bool pre_fault_allowed;
> -	struct hlist_head mmu_page_hash[KVM_NUM_MMU_PAGES];
> +	struct hlist_head *mmu_page_hash;
>  	struct list_head active_mmu_pages;
>  	/*
>  	 * A list of kvm_mmu_page structs that, if zapped, could possibly be
> @@ -1985,7 +1985,7 @@ void kvm_mmu_vendor_module_exit(void);
>  
>  void kvm_mmu_destroy(struct kvm_vcpu *vcpu);
>  int kvm_mmu_create(struct kvm_vcpu *vcpu);
> -void kvm_mmu_init_vm(struct kvm *kvm);
> +int kvm_mmu_init_vm(struct kvm *kvm);
>  void kvm_mmu_uninit_vm(struct kvm *kvm);
>  
>  void kvm_mmu_init_memslot_memory_attributes(struct kvm *kvm,
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 63bb77ee1bb1..6b9c72405860 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3880,6 +3880,18 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>  	return r;
>  }
>  
> +static int kvm_mmu_alloc_page_hash(struct kvm *kvm)
> +{
> +	typeof(kvm->arch.mmu_page_hash) h;
> +
> +	h = kcalloc(KVM_NUM_MMU_PAGES, sizeof(*h), GFP_KERNEL_ACCOUNT);
> +	if (!h)
> +		return -ENOMEM;
> +
> +	kvm->arch.mmu_page_hash = h;
> +	return 0;
> +}
> +
>  static int mmu_first_shadow_root_alloc(struct kvm *kvm)
>  {
>  	struct kvm_memslots *slots;
> @@ -6673,13 +6685,19 @@ static void kvm_mmu_zap_all_fast(struct kvm *kvm)
>  		kvm_tdp_mmu_zap_invalidated_roots(kvm, true);
>  }
>  
> -void kvm_mmu_init_vm(struct kvm *kvm)
> +int kvm_mmu_init_vm(struct kvm *kvm)
>  {
> +	int r;
> +
>  	kvm->arch.shadow_mmio_value = shadow_mmio_value;
>  	INIT_LIST_HEAD(&kvm->arch.active_mmu_pages);
>  	INIT_LIST_HEAD(&kvm->arch.possible_nx_huge_pages);
>  	spin_lock_init(&kvm->arch.mmu_unsync_pages_lock);
>  
> +	r = kvm_mmu_alloc_page_hash(kvm);
> +	if (r)
> +		return r;
> +
>  	if (tdp_mmu_enabled)
>  		kvm_mmu_init_tdp_mmu(kvm);
>  
> @@ -6690,6 +6708,7 @@ void kvm_mmu_init_vm(struct kvm *kvm)
>  
>  	kvm->arch.split_desc_cache.kmem_cache = pte_list_desc_cache;
>  	kvm->arch.split_desc_cache.gfp_zero = __GFP_ZERO;
> +	return 0;
>  }
>  
>  static void mmu_free_vm_memory_caches(struct kvm *kvm)
> @@ -6701,6 +6720,8 @@ static void mmu_free_vm_memory_caches(struct kvm *kvm)
>  
>  void kvm_mmu_uninit_vm(struct kvm *kvm)
>  {
> +	kfree(kvm->arch.mmu_page_hash);
> +
>  	if (tdp_mmu_enabled)
>  		kvm_mmu_uninit_tdp_mmu(kvm);
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index c841817a914a..4070f9d34521 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -12721,7 +12721,9 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  	if (ret)
>  		goto out;
>  
> -	kvm_mmu_init_vm(kvm);
> +	ret = kvm_mmu_init_vm(kvm);
> +	if (ret)
> +		goto out_cleanup_page_track;
>  
>  	ret = kvm_x86_call(vm_init)(kvm);
>  	if (ret)
> @@ -12774,6 +12776,7 @@ int kvm_arch_init_vm(struct kvm *kvm, unsigned long type)
>  
>  out_uninit_mmu:
>  	kvm_mmu_uninit_vm(kvm);
> +out_cleanup_page_track:
>  	kvm_page_track_cleanup(kvm);
>  out:
>  	return ret;
> -- 
> 2.49.0.472.ge94155a9ec-goog
> 

Reviewed-by: Vipin Sharma <vipinsh@...gle.com>