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
>
>
