[PATCH v5 13/15] KVM: arm64: Add HDBSS buffer size ioctl for dirty-bitmap mode
From: Tian Zheng <hidden>
Date: 2026-09-29 10:37:17
Also in:
kvm, kvmarm, linux-doc, linux-kselftest, lkml
Subsystem:
arm64 port (aarch64 architecture), kernel virtual machine (kvm), kernel virtual machine for arm64 (kvm/arm64), the rest · Maintainers:
Catalin Marinas, Will Deacon, Paolo Bonzini, Marc Zyngier, Oliver Upton, Linus Torvalds
In dirty-bitmap mode, a larger HDBSS buffer lets the guest dirty more pages between VM exits, at the cost of memory. The default of one page per vCPU is a reasonable starting point, so allow userspace to opt into a larger buffer via KVM_CAP_ARM_HDBSS_BUFFER_SIZE. The size is specified in bytes through KVM_ENABLE_CAP and must be a power of two in [PAGE_SIZE, SZ_2M]: the upper bound is the largest HDBSSBR_EL2.SZ encoding, and the power-of-two requirement matches the buddy allocator. It must be set before any vCPU is created: -EINVAL after vCPUs exist, -EBUSY if already set. The capability is rejected in dirty-ring mode, where the size is pinned by kvm_arch_dirty_ring_size_updated(). KVM_CHECK_EXTENSION returns the configured size for a VM, or SZ_2M when queried globally with a NULL kvm. Signed-off-by: Tian Zheng <redacted> --- arch/arm64/kvm/arm.c | 34 ++++++++++++++++++++++++++++++++++ include/uapi/linux/kvm.h | 1 + 2 files changed, 35 insertions(+)
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index 5a7046acb62e..eadbf3a88d31 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c@@ -19,6 +19,7 @@ #include <linux/kvm.h> #include <linux/kvm_irqfd.h> #include <linux/irqbypass.h> +#include <linux/log2.h> #include <linux/sched/stat.h> #include <linux/psci.h> #include <trace/events/kvm.h>
@@ -203,6 +204,31 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, r = 0; set_bit(KVM_ARCH_FLAG_EXIT_SEA, &kvm->arch.flags); break; + case KVM_CAP_ARM_HDBSS_BUFFER_SIZE: { + u64 size = cap->args[0]; + + if (!system_supports_hdbss()) + break; + if (kvm->dirty_ring_size) + break; + if (size < KVM_ARM_HDBSS_DEFAULT_SIZE || + size > KVM_ARM_HDBSS_MAX_SIZE) + break; + if (!is_power_of_2(size)) + break; + + mutex_lock(&kvm->lock); + if (kvm->created_vcpus) { + r = -EINVAL; + } else if (kvm->arch.hdbss_buffer_size) { + r = -EBUSY; + } else { + kvm->arch.hdbss_buffer_size = size; + r = 0; + } + mutex_unlock(&kvm->lock); + break; + } default: break; }
@@ -500,6 +526,14 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) else r = KVM_ARM_EAGER_SPLIT_CHUNK_SIZE_DEFAULT; break; + case KVM_CAP_ARM_HDBSS_BUFFER_SIZE: + if (!system_supports_hdbss()) + r = 0; + else if (kvm) + r = kvm->arch.hdbss_buffer_size ?: KVM_ARM_HDBSS_DEFAULT_SIZE; + else + r = KVM_ARM_HDBSS_MAX_SIZE; + break; case KVM_CAP_ARM_SUPPORTED_BLOCK_SIZES: r = kvm_supported_block_sizes(); break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index ac2d77d14963..59eb211494e4 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h@@ -999,6 +999,7 @@ struct kvm_enable_cap { #define KVM_CAP_S390_HPAGE_2G 249 #define KVM_CAP_PPC_COMPAT_CAPS 250 #define KVM_CAP_ARM_PMU_V3_STRICT 251 +#define KVM_CAP_ARM_HDBSS_BUFFER_SIZE 252 struct kvm_irq_routing_irqchip { __u32 irqchip; --
2.43.0