[PATCH v7 08/12] KVM: arm64: Add KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE

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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@xxxxxxxxx>
Signed-off-by: Ricardo Koller <ricarkol@xxxxxxxxxx>
---
 Documentation/virt/kvm/api.rst       | 28 ++++++++++++++++++++++++++
 arch/arm64/include/asm/kvm_host.h    | 15 ++++++++++++++
 arch/arm64/include/asm/kvm_pgtable.h | 18 +++++++++++++++++
 arch/arm64/kvm/arm.c                 | 30 ++++++++++++++++++++++++++++
 arch/arm64/kvm/mmu.c                 |  3 +++
 include/uapi/linux/kvm.h             |  2 ++
 6 files changed, 96 insertions(+)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 62de0768d6aa5..f8faa80d87057 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -8380,6 +8380,34 @@ 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 split chunk size.
+:Returns: 0 on success, -EINVAL if any memslot was already 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.  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.
+
+The chunk size needs to be a valid block size. The list of acceptable
+block sizes is exposed in KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES as a 64bit
+bitmap (each bit describing a block size). Setting
+KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE to 0 disables Eager Page Splitting;
+this is the default value.
+
 9. Known KVM API problems
 =========================
 
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index a1892a8f60323..b87da1ebc3454 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -158,6 +158,21 @@ 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 0
+	/*
+	 * Memory cache used to split
+	 * KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE worth of huge pages. It
+	 * is used to allocate stage2 page tables while splitting huge
+	 * pages. Note that the choice of EAGER_PAGE_SPLIT_CHUNK_SIZE
+	 * influences both the capacity of the split page cache, and
+	 * how often KVM reschedules. Be wary of raising CHUNK_SIZE
+	 * too high.
+	 *
+	 * Protected by kvm->slots_lock.
+	 */
+	struct kvm_mmu_memory_cache split_page_cache;
+	uint64_t split_page_chunk_size;
+
 	struct kvm_arch *arch;
 };
 
diff --git a/arch/arm64/include/asm/kvm_pgtable.h b/arch/arm64/include/asm/kvm_pgtable.h
index 32e5d42bf020f..889bd7afeb355 100644
--- a/arch/arm64/include/asm/kvm_pgtable.h
+++ b/arch/arm64/include/asm/kvm_pgtable.h
@@ -92,6 +92,24 @@ static inline bool kvm_level_supports_block_mapping(u32 level)
 	return level >= KVM_PGTABLE_MIN_BLOCK_LEVEL;
 }
 
+static inline u64 kvm_supported_block_sizes(void)
+{
+	u32 level = KVM_PGTABLE_MIN_BLOCK_LEVEL;
+	u64 res = 0;
+
+	for (; level < KVM_PGTABLE_MAX_LEVELS; level++)
+		res |= BIT(kvm_granule_shift(level));
+
+	return res;
+}
+
+static inline bool kvm_is_block_size_supported(u64 size)
+{
+	bool is_power_of_two = !((size) & ((size)-1));
+
+	return is_power_of_two && (size & kvm_supported_block_sizes());
+}
+
 /**
  * struct kvm_pgtable_mm_ops - Memory management callbacks.
  * @zalloc_page:		Allocate a single zeroed memory page.
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 3bd732eaf0872..34fd3c59a9b82 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -67,6 +67,7 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 			    struct kvm_enable_cap *cap)
 {
 	int r;
+	u64 new_cap;
 
 	if (cap->flags)
 		return -EINVAL;
@@ -91,6 +92,26 @@ 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:
+		new_cap = cap->args[0];
+
+		mutex_lock(&kvm->lock);
+		mutex_lock(&kvm->slots_lock);
+		/*
+		 * To keep things simple, allow changing the chunk
+		 * size only if there are no memslots already created.
+		 */
+		if (!kvm_are_all_memslots_empty(kvm)) {
+			r = -EINVAL;
+		} else if (new_cap && !kvm_is_block_size_supported(new_cap)) {
+			r = -EINVAL;
+		} else {
+			r = 0;
+			kvm->arch.mmu.split_page_chunk_size = new_cap;
+		}
+		mutex_unlock(&kvm->slots_lock);
+		mutex_unlock(&kvm->lock);
+		break;
 	default:
 		r = -EINVAL;
 		break;
@@ -288,6 +309,15 @@ 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;
+	case KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES:
+		r = kvm_supported_block_sizes();
+		break;
 	default:
 		r = 0;
 	}
diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
index a2800e5c42712..898985b09321a 100644
--- a/arch/arm64/kvm/mmu.c
+++ b/arch/arm64/kvm/mmu.c
@@ -756,6 +756,9 @@ 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 d77aef872a0a0..f18b48fcd25ba 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -1184,6 +1184,8 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_S390_PROTECTED_ASYNC_DISABLE 224
 #define KVM_CAP_DIRTY_LOG_RING_WITH_BITMAP 225
 #define KVM_CAP_PMU_EVENT_MASKED_EVENTS 226
+#define KVM_CAP_ARM_EAGER_SPLIT_CHUNK_SIZE 227
+#define KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES 228
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
2.40.0.577.gac1e443424-goog




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