[PATCH v5 07/15] KVM: arm64: Add HDBSS per-vCPU buffer management
From: Tian Zheng
Date: Tue Sep 29 2026 - 06:53:31 EST
From: Eillon <yezhenyu2@xxxxxxxxxx>
Each vCPU owns an HDBSS buffer, described by HDBSSBR_EL2 (base
address and encoded size) and advanced by HDBSSPROD_EL2 as hardware
appends entries. Tie the buffer lifetime to the vCPU: allocate at
vCPU creation, free at destruction. The buffer is allocated zeroed,
so a stale producer index can only ever observe invalid entries.
Registers are programmed whenever the vCPU owns a buffer, regardless
of whether HDBSS is enabled. The feature can be turned on
mid-KVM_RUN, and hardware dirty-state updates write to
HDBSSBR_EL2.BADDR without any fault, so the registers must already
be in place by then. HDBSSPROD_EL2 is preserved across context
switches and vCPU migration.
Two details: the buddy order is kept separate from the HDBSSBR_EL2.SZ
encoding, since the two only coincide on 4KB pages; and kvm_share_hyp()
is unwound in kvm_arch_vcpu_create() when the HDBSS allocation fails.
Signed-off-by: Eillon <yezhenyu2@xxxxxxxxxx>
Signed-off-by: Tian Zheng <zhengtian10@xxxxxxxxxx>
---
arch/arm64/include/asm/kvm_dirty_bit.h | 28 +++++++++++++
arch/arm64/include/asm/kvm_host.h | 13 ++++++
arch/arm64/include/asm/sysreg.h | 9 +++++
arch/arm64/kvm/Makefile | 1 +
arch/arm64/kvm/arm.c | 14 ++++++-
arch/arm64/kvm/dirty_bit.c | 55 ++++++++++++++++++++++++++
arch/arm64/kvm/hyp/vhe/switch.c | 17 ++++++++
arch/arm64/kvm/reset.c | 3 ++
8 files changed, 138 insertions(+), 2 deletions(-)
create mode 100644 arch/arm64/include/asm/kvm_dirty_bit.h
create mode 100644 arch/arm64/kvm/dirty_bit.c
diff --git a/arch/arm64/include/asm/kvm_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h
new file mode 100644
index 000000000000..fe703f02626b
--- /dev/null
+++ b/arch/arm64/include/asm/kvm_dirty_bit.h
@@ -0,0 +1,28 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Per-vCPU buffer management for HDBSS-based dirty page tracking.
+ *
+ * Copyright (C) 2026 Huawei Technologies Co., Ltd
+ * Author: Tian Zheng <zhengtian10@xxxxxxxxxx>
+ */
+
+#ifndef __ARM64_KVM_DIRTY_BIT_H__
+#define __ARM64_KVM_DIRTY_BIT_H__
+
+#include <asm/kvm_pgtable.h>
+#include <asm/sysreg.h>
+#include <linux/sizes.h>
+
+#define KVM_ARM_HDBSS_DEFAULT_SIZE PAGE_SIZE
+#define KVM_ARM_HDBSS_MAX_SIZE SZ_2M
+
+/* 0 means unconfigured, fall back to one page per vCPU. */
+static inline u32 kvm_hdbss_buffer_size(struct kvm *kvm)
+{
+ return kvm->arch.hdbss_buffer_size ?: KVM_ARM_HDBSS_DEFAULT_SIZE;
+}
+
+int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu);
+void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu);
+
+#endif /* __ARM64_KVM_DIRTY_BIT_H__ */
diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
index 86a4d6e50934..c8fc29f3db75 100644
--- a/arch/arm64/include/asm/kvm_host.h
+++ b/arch/arm64/include/asm/kvm_host.h
@@ -425,6 +425,9 @@ struct kvm_arch {
*/
struct kvm_protected_vm pkvm;
+ /* HDBSS: per-VM buffer size in bytes (0 = not configured, use default) */
+ u32 hdbss_buffer_size;
+
#ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
/* Nested virtualization info */
struct dentry *debugfs_nv_dentry;
@@ -844,6 +847,13 @@ struct vcpu_reset_state {
bool reset;
};
+struct vcpu_hdbss_state {
+ struct page *hdbss_pg; /* HDBSS buffer */
+ u64 hdbssbr_el2; /* programmed into the CPU on load */
+ u64 hdbssprod_el2; /* producer index, saved on put */
+ unsigned int buddy_order; /* allocation order for __free_pages() */
+};
+
struct vncr_tlb;
struct kvm_vcpu_arch {
@@ -951,6 +961,9 @@ struct kvm_vcpu_arch {
/* Hyp-readable copy of kvm_vcpu::pid */
pid_t pid;
+
+ /* HDBSS buffer state */
+ struct vcpu_hdbss_state hdbss;
};
/*
diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index 7aa08d59d494..7c71560b57e4 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -1039,6 +1039,15 @@
#define GCS_CAP(x) ((((unsigned long)x) & GCS_CAP_ADDR_MASK) | \
GCS_CAP_VALID_TOKEN)
+
+/*
+ * Definitions for the HDBSS feature
+ */
+#define HDBSSBR_EL2(baddr, sz) (((baddr) & HDBSSBR_EL2_BADDR_MASK) | \
+ FIELD_PREP(HDBSSBR_EL2_SZ_MASK, sz))
+
+#define HDBSSPROD_IDX(prod) FIELD_GET(HDBSSPROD_EL2_INDEX_MASK, prod)
+
/*
* Definitions for GICv5 instructions
*/
diff --git a/arch/arm64/kvm/Makefile b/arch/arm64/kvm/Makefile
index 59612d2f277c..ec2749af64fa 100644
--- a/arch/arm64/kvm/Makefile
+++ b/arch/arm64/kvm/Makefile
@@ -18,6 +18,7 @@ kvm-y += arm.o mmu.o mmio.o psci.o hypercalls.o pvtime.o \
guest.o debug.o reset.o sys_regs.o stacktrace.o \
vgic-sys-reg-v3.o fpsimd.o pkvm.o \
arch_timer.o trng.o vmid.o emulate-nested.o nested.o at.o \
+ dirty_bit.o \
vgic/vgic.o vgic/vgic-init.o \
vgic/vgic-irqfd.o vgic/vgic-v2.o \
vgic/vgic-v3.o vgic/vgic-v4.o \
diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
index d9ad765943d9..5ea4ac26995e 100644
--- a/arch/arm64/kvm/arm.c
+++ b/arch/arm64/kvm/arm.c
@@ -36,6 +36,7 @@
#include <asm/virt.h>
#include <asm/kvm_arm.h>
#include <asm/kvm_asm.h>
+#include <asm/kvm_dirty_bit.h>
#include <asm/kvm_emulate.h>
#include <asm/kvm_hyp.h>
#include <asm/kvm_mmu.h>
@@ -580,10 +581,19 @@ int kvm_arch_vcpu_create(struct kvm_vcpu *vcpu)
}
err = kvm_share_hyp(vcpu, vcpu + 1);
- if (err)
+ if (err) {
kvm_vgic_vcpu_destroy(vcpu);
+ return err;
+ }
- return err;
+ err = kvm_arm_vcpu_alloc_hdbss(vcpu);
+ if (err) {
+ kvm_unshare_hyp(vcpu, vcpu + 1);
+ kvm_vgic_vcpu_destroy(vcpu);
+ return err;
+ }
+
+ return 0;
}
void kvm_arch_vcpu_postcreate(struct kvm_vcpu *vcpu)
diff --git a/arch/arm64/kvm/dirty_bit.c b/arch/arm64/kvm/dirty_bit.c
new file mode 100644
index 000000000000..f9aeb9f34ad0
--- /dev/null
+++ b/arch/arm64/kvm/dirty_bit.c
@@ -0,0 +1,55 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Per-vCPU HDBSS buffer management.
+ *
+ * Copyright (C) 2026 Huawei Technologies Co., Ltd
+ * Author: Tian Zheng <zhengtian10@xxxxxxxxxx>
+ */
+
+#include <asm/kvm_dirty_bit.h>
+#include <asm/kvm_mmu.h>
+#include <asm/sysreg.h>
+#include <linux/gfp.h>
+#include <linux/kconfig.h>
+#include <linux/log2.h>
+#include <linux/mm.h>
+
+int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu)
+{
+ struct page *hdbss_pg;
+ u32 size;
+ unsigned int buddy_order;
+ u32 sz_encoded;
+
+ if (vcpu->arch.hdbss.hdbss_pg || !system_supports_hdbss())
+ return 0;
+
+ size = kvm_hdbss_buffer_size(vcpu->kvm);
+
+ buddy_order = get_order(size);
+ sz_encoded = ilog2(size) - 12;
+
+ hdbss_pg = alloc_pages(GFP_KERNEL_ACCOUNT | __GFP_ZERO, buddy_order);
+ if (!hdbss_pg)
+ return -ENOMEM;
+
+ vcpu->arch.hdbss = (struct vcpu_hdbss_state) {
+ .hdbss_pg = hdbss_pg,
+ .hdbssbr_el2 = HDBSSBR_EL2(page_to_phys(hdbss_pg), sz_encoded),
+ .hdbssprod_el2 = 0,
+ .buddy_order = buddy_order,
+ };
+
+ return 0;
+}
+
+void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu)
+{
+ if (!vcpu->arch.hdbss.hdbss_pg)
+ return;
+
+ __free_pages(vcpu->arch.hdbss.hdbss_pg, vcpu->arch.hdbss.buddy_order);
+
+ vcpu->arch.hdbss.hdbss_pg = NULL;
+ vcpu->arch.hdbss.hdbssbr_el2 = 0;
+}
diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index 7875911c0506..922fc9260e11 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -19,6 +19,7 @@
#include <asm/cpufeature.h>
#include <asm/kprobes.h>
#include <asm/kvm_asm.h>
+#include <asm/kvm_dirty_bit.h>
#include <asm/kvm_emulate.h>
#include <asm/kvm_hyp.h>
#include <asm/kvm_mmu.h>
@@ -219,6 +220,17 @@ static void __vcpu_put_deactivate_traps(struct kvm_vcpu *vcpu)
local_irq_restore(flags);
}
+static void __load_hdbss(struct kvm_vcpu *vcpu)
+{
+ if (!vcpu->arch.hdbss.hdbss_pg)
+ return;
+
+ write_sysreg_s(vcpu->arch.hdbss.hdbssbr_el2, SYS_HDBSSBR_EL2);
+ write_sysreg_s(vcpu->arch.hdbss.hdbssprod_el2, SYS_HDBSSPROD_EL2);
+
+ isb();
+}
+
void kvm_vcpu_load_vhe(struct kvm_vcpu *vcpu)
{
host_data_ptr(host_ctxt)->__hyp_running_vcpu = vcpu;
@@ -226,10 +238,15 @@ void kvm_vcpu_load_vhe(struct kvm_vcpu *vcpu)
__vcpu_load_switch_sysregs(vcpu);
__vcpu_load_activate_traps(vcpu);
__load_stage2(vcpu->arch.hw_mmu);
+ __load_hdbss(vcpu);
}
void kvm_vcpu_put_vhe(struct kvm_vcpu *vcpu)
{
+ /* Saved under the same ownership condition as __load_hdbss(). */
+ if (vcpu->arch.hdbss.hdbss_pg)
+ vcpu->arch.hdbss.hdbssprod_el2 = read_sysreg_s(SYS_HDBSSPROD_EL2);
+
__vcpu_put_deactivate_traps(vcpu);
__vcpu_put_switch_sysregs(vcpu);
diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
index 10eb7249aa9e..05ebe304830e 100644
--- a/arch/arm64/kvm/reset.c
+++ b/arch/arm64/kvm/reset.c
@@ -25,6 +25,7 @@
#include <asm/ptrace.h>
#include <asm/kvm_arm.h>
#include <asm/kvm_asm.h>
+#include <asm/kvm_dirty_bit.h>
#include <asm/kvm_emulate.h>
#include <asm/kvm_mmu.h>
#include <asm/kvm_nested.h>
@@ -149,6 +150,8 @@ void kvm_arm_vcpu_destroy(struct kvm_vcpu *vcpu)
free_page((unsigned long)vcpu->arch.ctxt.vncr_array);
kfree(vcpu->arch.vncr_tlb);
kfree(vcpu->arch.ccsidr);
+
+ kvm_arm_vcpu_free_hdbss(vcpu);
}
static void kvm_vcpu_reset_sve(struct kvm_vcpu *vcpu)
--
2.43.0