Re: [PATCH 6/8] KVM: x86/mmu: Check for usable TDP MMU root while holding mmu_lock for read

[Xu Yilun]

On Wed, Jan 10, 2024 at 06:00:46PM -0800, Sean Christopherson wrote:
> When allocating a new TDP MMU root, check for a usable root while holding
> mmu_lock for read and only acquire mmu_lock for write if a new root needs
> to be created.  There is no need to serialize other MMU operations if a
> vCPU is simply grabbing a reference to an existing root, holding mmu_lock
> for write is "necessary" (spoiler alert, it's not strictly necessary) only
> to ensure KVM doesn't end up with duplicate roots.
> 
> Allowing vCPUs to get "new" roots in parallel is beneficial to VM boot and
> to setups that frequently delete memslots, i.e. which force all vCPUs to
> reload all roots.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> ---
>  arch/x86/kvm/mmu/mmu.c     |  8 ++---
>  arch/x86/kvm/mmu/tdp_mmu.c | 60 +++++++++++++++++++++++++++++++-------
>  arch/x86/kvm/mmu/tdp_mmu.h |  2 +-
>  3 files changed, 55 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 3c844e428684..ea18aca23196 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3693,15 +3693,15 @@ static int mmu_alloc_direct_roots(struct kvm_vcpu *vcpu)
>  	unsigned i;
>  	int r;
>  
> +	if (tdp_mmu_enabled)
> +		return kvm_tdp_mmu_alloc_root(vcpu);
> +
>  	write_lock(&vcpu->kvm->mmu_lock);
>  	r = make_mmu_pages_available(vcpu);
>  	if (r < 0)
>  		goto out_unlock;
>  
> -	if (tdp_mmu_enabled) {
> -		root = kvm_tdp_mmu_get_vcpu_root_hpa(vcpu);
> -		mmu->root.hpa = root;
> -	} else if (shadow_root_level >= PT64_ROOT_4LEVEL) {
> +	if (shadow_root_level >= PT64_ROOT_4LEVEL) {
>  		root = mmu_alloc_root(vcpu, 0, 0, shadow_root_level);
>  		mmu->root.hpa = root;
>  	} else if (shadow_root_level == PT32E_ROOT_LEVEL) {
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index e0a8343f66dc..9a8250a14fc1 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -223,21 +223,52 @@ static void tdp_mmu_init_child_sp(struct kvm_mmu_page *child_sp,
>  	tdp_mmu_init_sp(child_sp, iter->sptep, iter->gfn, role);
>  }
>  
> -hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
> +static struct kvm_mmu_page *kvm_tdp_mmu_try_get_root(struct kvm_vcpu *vcpu)
>  {
>  	union kvm_mmu_page_role role = vcpu->arch.mmu->root_role;
> +	int as_id = kvm_mmu_role_as_id(role);
>  	struct kvm *kvm = vcpu->kvm;
>  	struct kvm_mmu_page *root;
>  
> -	lockdep_assert_held_write(&kvm->mmu_lock);
> -
> -	/* Check for an existing root before allocating a new one. */
> -	for_each_valid_tdp_mmu_root(kvm, root, kvm_mmu_role_as_id(role)) {
> -		if (root->role.word == role.word &&
> -		    kvm_tdp_mmu_get_root(root))
> -			goto out;
> +	for_each_valid_tdp_mmu_root_yield_safe(kvm, root, as_id) {

No lock yielding attempt in this loop, why change to _yield_safe version?

Thanks,
Yilun

> +		if (root->role.word == role.word)
> +			return root;
>  	}
>  
> +	return NULL;
> +}
> +
> +int kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu)
> +{
> +	struct kvm_mmu *mmu = vcpu->arch.mmu;
> +	union kvm_mmu_page_role role = mmu->root_role;
> +	struct kvm *kvm = vcpu->kvm;
> +	struct kvm_mmu_page *root;
> +
> +	/*
> +	 * Check for an existing root while holding mmu_lock for read to avoid
> +	 * unnecessary serialization if multiple vCPUs are loading a new root.
> +	 * E.g. when bringing up secondary vCPUs, KVM will already have created
> +	 * a valid root on behalf of the primary vCPU.
> +	 */
> +	read_lock(&kvm->mmu_lock);
> +	root = kvm_tdp_mmu_try_get_root(vcpu);
> +	read_unlock(&kvm->mmu_lock);
> +
> +	if (root)
> +		goto out;
> +
> +	write_lock(&kvm->mmu_lock);
> +
> +	/*
> +	 * Recheck for an existing root after acquiring mmu_lock for write.  It
> +	 * is possible a new usable root was created between dropping mmu_lock
> +	 * (for read) and acquiring it for write.
> +	 */
> +	root = kvm_tdp_mmu_try_get_root(vcpu);
> +	if (root)
> +		goto out_unlock;
> +
>  	root = tdp_mmu_alloc_sp(vcpu);
>  	tdp_mmu_init_sp(root, NULL, 0, role);
>  
> @@ -254,8 +285,17 @@ hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu)
>  	list_add_rcu(&root->link, &kvm->arch.tdp_mmu_roots);
>  	spin_unlock(&kvm->arch.tdp_mmu_pages_lock);
>  
> +out_unlock:
> +	write_unlock(&kvm->mmu_lock);
>  out:
> -	return __pa(root->spt);
> +	/*
> +	 * Note, KVM_REQ_MMU_FREE_OBSOLETE_ROOTS will prevent entering the guest
> +	 * and actually consuming the root if it's invalidated after dropping
> +	 * mmu_lock, and the root can't be freed as this vCPU holds a reference.
> +	 */
> +	mmu->root.hpa = __pa(root->spt);
> +	mmu->root.pgd = 0;
> +	return 0;
>  }
>  
>  static void handle_changed_spte(struct kvm *kvm, int as_id, gfn_t gfn,
> @@ -917,7 +957,7 @@ void kvm_tdp_mmu_zap_invalidated_roots(struct kvm *kvm)
>   * the VM is being destroyed).
>   *
>   * Note, kvm_tdp_mmu_zap_invalidated_roots() is gifted the TDP MMU's reference.
> - * See kvm_tdp_mmu_get_vcpu_root_hpa().
> + * See kvm_tdp_mmu_alloc_root().
>   */
>  void kvm_tdp_mmu_invalidate_all_roots(struct kvm *kvm)
>  {
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.h b/arch/x86/kvm/mmu/tdp_mmu.h
> index 20d97aa46c49..6e1ea04ca885 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.h
> +++ b/arch/x86/kvm/mmu/tdp_mmu.h
> @@ -10,7 +10,7 @@
>  void kvm_mmu_init_tdp_mmu(struct kvm *kvm);
>  void kvm_mmu_uninit_tdp_mmu(struct kvm *kvm);
>  
> -hpa_t kvm_tdp_mmu_get_vcpu_root_hpa(struct kvm_vcpu *vcpu);
> +int kvm_tdp_mmu_alloc_root(struct kvm_vcpu *vcpu);
>  
>  __must_check static inline bool kvm_tdp_mmu_get_root(struct kvm_mmu_page *root)
>  {
> -- 
> 2.43.0.275.g3460e3d667-goog
> 
>