[PATCH v2 08/12] KVM: arm64: Add KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE

[Ricardo Koller <ricarkol@google.com>]

Add a capability for userspace to specify the eager split chunk size.
The chunk size specifies how many pages to break at a time, using a
single allocation. Bigger the chunk size, more pages need to be
allocated ahead of time.

Suggested-by: Oliver Upton <oliver.upton@linux.dev>
Signed-off-by: Ricardo Koller <ricarkol@google.com>
---
 Documentation/virt/kvm/api.rst    | 26 ++++++++++++++++++++++++++
 arch/arm64/include/asm/kvm_host.h |  2 ++
 arch/arm64/kvm/arm.c              | 22 ++++++++++++++++++++++
 arch/arm64/kvm/mmu.c              |  3 +++
 include/uapi/linux/kvm.h          |  1 +
 5 files changed, 54 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 9807b05a1b57..a9332e331cce 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8284,6 +8284,32 @@ structure.
 When getting the Modified Change Topology Report value, the attr->addr
 must point to a byte where the value will be stored or retrieved from.
 
+8.40 KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE
+---------------------------------------
+
+:Capability: KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE
+:Architectures: arm64
+:Type: vm
+:Parameters: arg[0] is the new chunk size.
+:Returns: 0 on success, -EINVAL if any memslot has been created.
+
+This capability sets the chunk size used in Eager Page Splitting.
+
+Eager Page Splitting improves the performance of dirty-logging (used
+in live migrations) when guest memory is backed by huge-pages.  This
+optimization is enabled by default on arm64. It avoids splitting
+huge-pages (into PAGE_SIZE pages) on fault, by doing it eagerly when
+enabling dirty logging (with the KVM_MEM_LOG_DIRTY_PAGES flag for a
+memory region), or when using KVM_CLEAR_DIRTY_LOG.
+
+The chunk size specifies how many pages to break at a time, using a
+single allocation for each chunk. Bigger the chunk size, more pages
+need to be allocated ahead of time. A good heuristic is to pick the
+size of the huge-pages as the chunk size.
+
+If the chunk size (arg[0]) is zero, then no eager page splitting is
+performed. The default value PMD size (e.g., 2M when PAGE_SIZE is 4K).
+
 9. Known KVM API problems
 =========================
 
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 35a159d131b5..a69a815719cf 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -153,6 +153,8 @@ struct kvm_s2_mmu {
 	/* The last vcpu id that ran on each physical CPU */
 	int __percpu *last_vcpu_ran;
 
+#define KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT	PMD_SIZE
+
 	struct kvm_arch *arch;
 };
 
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 9c5573bc4614..c80617ced599 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -101,6 +101,22 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		r = 0;
 		set_bit(KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED, &kvm->arch.flags);
 		break;
+	case KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE:
+		mutex_lock(&kvm->lock);
+		mutex_lock(&kvm->slots_lock);
+		/*
+		 * To keep things simple, allow changing the chunk
+		 * size only if there are no memslots created.
+		 */
+		if (!kvm_are_all_memslots_empty(kvm)) {
+			r = -EINVAL;
+		} else {
+			r = 0;
+			kvm->arch.mmu.split_page_chunk_size = cap->args[0];
+		}
+		mutex_unlock(&kvm->slots_lock);
+		mutex_unlock(&kvm->lock);
+		break;
 	default:
 		r = -EINVAL;
 		break;
@@ -298,6 +314,12 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 	case KVM_CAP_ARM_PTRAUTH_GENERIC:
 		r = system_has_full_ptr_auth();
 		break;
+	case KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE:
+		if (kvm)
+			r = kvm->arch.mmu.split_page_chunk_size;
+		else
+			r = KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT;
+		break;
 	default:
 		r = 0;
 	}
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index 812633a75e74..e2ada6588017 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -755,6 +755,9 @@ int kvm_init_stage2_mmu(struct kvm *kvm, struct kvm_s2_mmu *mmu, unsigned long t
 	for_each_possible_cpu(cpu)
 		*per_cpu_ptr(mmu->last_vcpu_ran, cpu) = -1;
 
+	mmu->split_page_cache.gfp_zero = __GFP_ZERO;
+	mmu->split_page_chunk_size = KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT;
+
 	mmu->pgt = pgt;
 	mmu->pgd_phys = __pa(pgt->pgd);
 	return 0;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 55155e262646..02e05f7918e2 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1175,6 +1175,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_DIRTY_LOG_RING_ACQ_REL 223
 #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
 #define KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP 225
+#define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 226
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.39.1.519.gcb327c4b5f-goog