Subject: Re: [PATCH v3 4/5] KVM: arm64: Enable HDBSS support and handle HDBSSF events

From: Leonardo Bras <[email-masked]>
Date: Thu, 26 Mar 2026 14:31:38 +0000

On Wed, Mar 25, 2026 at 06:05:26PM +0000, Leonardo Bras wrote:
> Hello Tian,
> 
> I am currently working on HACDBS enablement(which will be rebased on top of 
> this patchset) and due to the fact HACDBS and HDBSS are kind of 
> complementary I will sometimes come with some questions for issues I have 
> faced myself on that part. :)
> 
> (see below)
> 
> On Wed, Feb 25, 2026 at 12:04:20PM +0800, Tian Zheng wrote:
> > From: eillon <[email-masked]>
> > 
> > HDBSS is enabled via an ioctl from userspace (e.g. QEMU) at the start of
> > migration. This feature is only supported in VHE mode.
> > 
> > Initially, S2 PTEs doesn't contain the DBM attribute. During migration,
> > write faults are handled by user_mem_abort, which relaxes permissions
> > and adds the DBM bit when HDBSS is active. Once DBM is set, subsequent
> > writes no longer trap, as the hardware automatically transitions the page
> > from writable-clean to writable-dirty.
> > 
> > KVM does not scan S2 page tables to consume DBM. Instead, when HDBSS is
> > enabled, the hardware observes the clean->dirty transition and records
> > the corresponding page into the HDBSS buffer.
> > 
> > During sync_dirty_log, KVM kicks all vCPUs to force VM-Exit, ensuring
> > that check_vcpu_requests flushes the HDBSS buffer and propagates the
> > accumulated dirty information into the userspace-visible dirty bitmap.
> > 
> > Add fault handling for HDBSS including buffer full, external abort, and
> > general protection fault (GPF).
> > 
> > Signed-off-by: eillon <[email-masked]>
> > Signed-off-by: Tian Zheng <[email-masked]>
> > ---
> >  arch/arm64/include/asm/esr.h      |   5 ++
> >  arch/arm64/include/asm/kvm_host.h |  17 +++++
> >  arch/arm64/include/asm/kvm_mmu.h  |   1 +
> >  arch/arm64/include/asm/sysreg.h   |  11 ++++
> >  arch/arm64/kvm/arm.c              | 102 ++++++++++++++++++++++++++++++
> >  arch/arm64/kvm/hyp/vhe/switch.c   |  19 ++++++
> >  arch/arm64/kvm/mmu.c              |  70 ++++++++++++++++++++
> >  arch/arm64/kvm/reset.c            |   3 +
> >  8 files changed, 228 insertions(+)
> > 
> > diff --git a/arch/arm64/include/asm/esr.h b/arch/arm64/include/asm/esr.h
> > index 81c17320a588..2e6b679b5908 100644
> > --- a/arch/arm64/include/asm/esr.h
> > +++ b/arch/arm64/include/asm/esr.h
> > @@ -437,6 +437,11 @@
> >  #ifndef __ASSEMBLER__
> >  #include <asm/types.h>
> > 
> > +static inline bool esr_iss2_is_hdbssf(unsigned long esr)
> > +{
> > +	return ESR_ELx_ISS2(esr) & ESR_ELx_HDBSSF;
> > +}
> > +
> >  static inline unsigned long esr_brk_comment(unsigned long esr)
> >  {
> >  	return esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
> > diff --git a/arch/arm64/include/asm/kvm_host.h b/arch/arm64/include/asm/kvm_host.h
> > index 5d5a3bbdb95e..57ee6b53e061 100644
> > --- a/arch/arm64/include/asm/kvm_host.h
> > +++ b/arch/arm64/include/asm/kvm_host.h
> > @@ -55,12 +55,17 @@
> >  #define KVM_REQ_GUEST_HYP_IRQ_PENDING	KVM_ARCH_REQ(9)
> >  #define KVM_REQ_MAP_L1_VNCR_EL2		KVM_ARCH_REQ(10)
> >  #define KVM_REQ_VGIC_PROCESS_UPDATE	KVM_ARCH_REQ(11)
> > +#define KVM_REQ_FLUSH_HDBSS			KVM_ARCH_REQ(12)
> > 
> >  #define KVM_DIRTY_LOG_MANUAL_CAPS   (KVM_DIRTY_LOG_MANUAL_PROTECT_ENABLE | \
> >  				     KVM_DIRTY_LOG_INITIALLY_SET)
> > 
> >  #define KVM_HAVE_MMU_RWLOCK
> > 
> > +/* HDBSS entry field definitions */
> > +#define HDBSS_ENTRY_VALID BIT(0)
> > +#define HDBSS_ENTRY_IPA GENMASK_ULL(55, 12)
> > +
> >  /*
> >   * Mode of operation configurable with kvm-arm.mode early param.
> >   * See Documentation/admin-guide/kernel-parameters.txt for more information.
> > @@ -84,6 +89,7 @@ int __init kvm_arm_init_sve(void);
> >  u32 __attribute_const__ kvm_target_cpu(void);
> >  void kvm_reset_vcpu(struct kvm_vcpu *vcpu);
> >  void kvm_arm_vcpu_destroy(struct kvm_vcpu *vcpu);
> > +void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu);
> > 
> >  struct kvm_hyp_memcache {
> >  	phys_addr_t head;
> > @@ -405,6 +411,8 @@ struct kvm_arch {
> >  	 * the associated pKVM instance in the hypervisor.
> >  	 */
> >  	struct kvm_protected_vm pkvm;
> > +
> > +	bool enable_hdbss;
> >  };
> > 
> >  struct kvm_vcpu_fault_info {
> > @@ -816,6 +824,12 @@ struct vcpu_reset_state {
> >  	bool		reset;
> >  };
> > 
> > +struct vcpu_hdbss_state {
> > +	phys_addr_t base_phys;
> > +	u32 size;
> > +	u32 next_index;
> > +};
> > +
> >  struct vncr_tlb;
> > 
> >  struct kvm_vcpu_arch {
> > @@ -920,6 +934,9 @@ struct kvm_vcpu_arch {
> > 
> >  	/* Per-vcpu TLB for VNCR_EL2 -- NULL when !NV */
> >  	struct vncr_tlb	*vncr_tlb;
> > +
> > +	/* HDBSS registers info */
> > +	struct vcpu_hdbss_state hdbss;
> >  };
> > 
> >  /*
> > diff --git a/arch/arm64/include/asm/kvm_mmu.h b/arch/arm64/include/asm/kvm_mmu.h
> > index d968aca0461a..3fea8cfe8869 100644
> > --- a/arch/arm64/include/asm/kvm_mmu.h
> > +++ b/arch/arm64/include/asm/kvm_mmu.h
> > @@ -183,6 +183,7 @@ int kvm_phys_addr_ioremap(struct kvm *kvm, phys_addr_t guest_ipa,
> > 
> >  int kvm_handle_guest_sea(struct kvm_vcpu *vcpu);
> >  int kvm_handle_guest_abort(struct kvm_vcpu *vcpu);
> > +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu);
> > 
> >  phys_addr_t kvm_mmu_get_httbr(void);
> >  phys_addr_t kvm_get_idmap_vector(void);
> > diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
> > index f4436ecc630c..d11f4d0dd4e7 100644
> > --- a/arch/arm64/include/asm/sysreg.h
> > +++ b/arch/arm64/include/asm/sysreg.h
> > @@ -1039,6 +1039,17 @@
> > 
> >  #define GCS_CAP(x)	((((unsigned long)x) & GCS_CAP_ADDR_MASK) | \
> >  					       GCS_CAP_VALID_TOKEN)
> > +
> > +/*
> > + * Definitions for the HDBSS feature
> > + */
> > +#define HDBSS_MAX_SIZE		HDBSSBR_EL2_SZ_2MB
> > +
> > +#define HDBSSBR_EL2(baddr, sz)	(((baddr) & GENMASK(55, 12 + sz)) | \
> > +				 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/arm.c b/arch/arm64/kvm/arm.c
> > index 29f0326f7e00..d64da05e25c4 100644
> > --- a/arch/arm64/kvm/arm.c
> > +++ b/arch/arm64/kvm/arm.c
> > @@ -125,6 +125,87 @@ int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
> >  	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
> >  }
> > 
> > +void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu)
> > +{
> > +	struct page *hdbss_pg;
> > +
> > +	hdbss_pg = phys_to_page(vcpu->arch.hdbss.base_phys);
> > +	if (hdbss_pg)
> > +		__free_pages(hdbss_pg, vcpu->arch.hdbss.size);
> > +
> > +	vcpu->arch.hdbss.size = 0;
> > +}
> > +
> > +static int kvm_cap_arm_enable_hdbss(struct kvm *kvm,
> > +				    struct kvm_enable_cap *cap)
> > +{
> > +	unsigned long i;
> > +	struct kvm_vcpu *vcpu;
> > +	struct page *hdbss_pg = NULL;
> > +	__u64 size = cap->args[0];
> > +	bool enable = cap->args[1] ? true : false;
> > +
> > +	if (!system_supports_hdbss())
> > +		return -EINVAL;
> > +
> > +	if (size > HDBSS_MAX_SIZE)
> > +		return -EINVAL;
> > +
> > +	if (!enable && !kvm->arch.enable_hdbss) /* Already Off */
> > +		return 0;
> > +
> > +	if (enable && kvm->arch.enable_hdbss) /* Already On, can't set size */
> > +		return -EINVAL;
> > +
> > +	if (!enable) { /* Turn it off */
> > +		kvm->arch.mmu.vtcr &= ~(VTCR_EL2_HD | VTCR_EL2_HDBSS | VTCR_EL2_HA);
> > +
> > +		kvm_for_each_vcpu(i, vcpu, kvm) {
> > +			/* Kick vcpus to flush hdbss buffer. */
> > +			kvm_vcpu_kick(vcpu);
> > +
> > +			kvm_arm_vcpu_free_hdbss(vcpu);
> > +		}
> > +
> > +		kvm->arch.enable_hdbss = false;
> > +
> > +		return 0;
> > +	}
> > +
> > +	/* Turn it on */
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		hdbss_pg = alloc_pages(GFP_KERNEL_ACCOUNT, size);
> > +		if (!hdbss_pg)
> > +			goto error_alloc;
> > +
> > +		vcpu->arch.hdbss = (struct vcpu_hdbss_state) {
> > +			.base_phys = page_to_phys(hdbss_pg),
> > +			.size = size,
> > +			.next_index = 0,
> > +		};
> > +	}
> > +
> > +	kvm->arch.enable_hdbss = true;
> > +	kvm->arch.mmu.vtcr |= VTCR_EL2_HD | VTCR_EL2_HDBSS | VTCR_EL2_HA;
> > +
> > +	/*
> > +	 * We should kick vcpus out of guest mode here to load new
> > +	 * vtcr value to vtcr_el2 register when re-enter guest mode.
> > +	 */
> > +	kvm_for_each_vcpu(i, vcpu, kvm)
> > +		kvm_vcpu_kick(vcpu);
> > +
> > +	return 0;
> > +
> > +error_alloc:
> > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > +		if (vcpu->arch.hdbss.base_phys)
> > +			kvm_arm_vcpu_free_hdbss(vcpu);
> > +	}
> > +
> > +	return -ENOMEM;
> > +}
> > +
> >  int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
> >  			    struct kvm_enable_cap *cap)
> >  {
> > @@ -182,6 +263,11 @@ 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_HW_DIRTY_STATE_TRACK:
> > +		mutex_lock(&kvm->lock);
> > +		r = kvm_cap_arm_enable_hdbss(kvm, cap);
> > +		mutex_unlock(&kvm->lock);
> > +		break;
> >  	default:
> >  		break;
> >  	}
> > @@ -471,6 +557,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
> >  			r = kvm_supports_cacheable_pfnmap();
> >  		break;
> > 
> > +	case KVM_CAP_ARM_HW_DIRTY_STATE_TRACK:
> > +		r = system_supports_hdbss();
> > +		break;
> >  	default:
> >  		r = 0;
> >  	}
> > @@ -1120,6 +1209,9 @@ static int check_vcpu_requests(struct kvm_vcpu *vcpu)
> >  		if (kvm_dirty_ring_check_request(vcpu))
> >  			return 0;
> > 
> > +		if (kvm_check_request(KVM_REQ_FLUSH_HDBSS, vcpu))
> > +			kvm_flush_hdbss_buffer(vcpu);
> > +
> >  		check_nested_vcpu_requests(vcpu);
> >  	}
> > 
> > @@ -1898,7 +1990,17 @@ long kvm_arch_vcpu_unlocked_ioctl(struct file *filp, unsigned int ioctl,
> > 
> >  void kvm_arch_sync_dirty_log(struct kvm *kvm, struct kvm_memory_slot *memslot)
> >  {
> > +	/*
> > +	 * Flush all CPUs' dirty log buffers to the dirty_bitmap.  Called
> > +	 * before reporting dirty_bitmap to userspace. Send a request with
> > +	 * KVM_REQUEST_WAIT to flush buffer synchronously.
> > +	 */
> > +	struct kvm_vcpu *vcpu;
> > +
> > +	if (!kvm->arch.enable_hdbss)
> > +		return;
> > 
> > +	kvm_make_all_cpus_request(kvm, KVM_REQ_FLUSH_HDBSS);
> >  }
> > 
> >  static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
> > diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> > index 9db3f11a4754..600cbc4f8ae9 100644
> > --- a/arch/arm64/kvm/hyp/vhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> > @@ -213,6 +213,23 @@ static void __vcpu_put_deactivate_traps(struct kvm_vcpu *vcpu)
> >  	local_irq_restore(flags);
> >  }
> > 
> > +static void __load_hdbss(struct kvm_vcpu *vcpu)
> > +{
> > +	struct kvm *kvm = vcpu->kvm;
> > +	u64 br_el2, prod_el2;
> > +
> > +	if (!kvm->arch.enable_hdbss)
> > +		return;
> > +
> > +	br_el2 = HDBSSBR_EL2(vcpu->arch.hdbss.base_phys, vcpu->arch.hdbss.size);
> > +	prod_el2 = vcpu->arch.hdbss.next_index;
> > +
> > +	write_sysreg_s(br_el2, SYS_HDBSSBR_EL2);
> > +	write_sysreg_s(prod_el2, SYS_HDBSSPROD_EL2);
> > +
> > +	isb();
> > +}
> > +
> 
> I see in the code below you trust that the tracking will happen with 
> PAGE_SIZE granularity (you track with PAGE_SHIFT).

Oh, that was misleading :\
The mentioned routine is not wrong AFAICS, but without the patch I sent, if 
the VMM does not manually set eager splitting, and is using some sort of 
hugepages (even transparent, without noticing) it could end up sending just 
PAGE_SIZE instead of the whole hugepage size, which breaks migration. 

Does that make sense?

Thanks!
Leo

> 
> That may be a problem when we have guest memory backed by hugepages or 
> transparent huge pages. 
> 
> When we are using HDBSS, there is no fault happening, so we have no way of 
> doing on-demand block splitting, so we need to make use of eager block 
> splitting, _before_ we start to track anything, or else we may have 
> different-sized pages in the HDBSS buffer, which is harder to deal with.
> 
> Suggestion: do the eager splitting before we enable HDBSS. 
> 
> For this to happen, we have to enable the EAGER_SPLIT_CHUNK_SIZE 
> capability, which can only be enabled when all memslots are empty.
> 
> I suggest doing that at kvm_init_stage2_mmu(), and checking if HDBSS is 
> in which case we set mmu->split_page_chunk_size to PAGESIZE.
> 
> I will send a patch you can put before this one to make sure it works :)
> 
> Thanks!
> Leo
> 
> 
> >  void kvm_vcpu_load_vhe(struct kvm_vcpu *vcpu)
> >  {
> >  	host_data_ptr(host_ctxt)->__hyp_running_vcpu = vcpu;
> > @@ -220,10 +237,12 @@ 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, vcpu->arch.hw_mmu->arch);
> > +	__load_hdbss(vcpu);
> >  }
> > 
> >  void kvm_vcpu_put_vhe(struct kvm_vcpu *vcpu)
> >  {
> > +	kvm_flush_hdbss_buffer(vcpu);
> >  	__vcpu_put_deactivate_traps(vcpu);
> >  	__vcpu_put_switch_sysregs(vcpu);
> > 
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 070a01e53fcb..42b0710a16ce 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -1896,6 +1896,9 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >  	if (writable)
> >  		prot |= KVM_PGTABLE_PROT_W;
> > 
> > +	if (writable && kvm->arch.enable_hdbss && logging_active)
> > +		prot |= KVM_PGTABLE_PROT_DBM;
> > +
> >  	if (exec_fault)
> >  		prot |= KVM_PGTABLE_PROT_X;
> > 
> > @@ -2033,6 +2036,70 @@ int kvm_handle_guest_sea(struct kvm_vcpu *vcpu)
> >  	return 0;
> >  }
> > 
> > +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu)
> > +{
> > +	int idx, curr_idx;
> > +	u64 br_el2;
> > +	u64 *hdbss_buf;
> > +	struct kvm *kvm = vcpu->kvm;
> > +
> > +	if (!kvm->arch.enable_hdbss)
> > +		return;
> > +
> > +	curr_idx = HDBSSPROD_IDX(read_sysreg_s(SYS_HDBSSPROD_EL2));
> > +	br_el2 = HDBSSBR_EL2(vcpu->arch.hdbss.base_phys, vcpu->arch.hdbss.size);
> > +
> > +	/* Do nothing if HDBSS buffer is empty or br_el2 is NULL */
> > +	if (curr_idx == 0 || br_el2 == 0)
> > +		return;
> > +
> > +	hdbss_buf = page_address(phys_to_page(vcpu->arch.hdbss.base_phys));
> > +	if (!hdbss_buf)
> > +		return;
> > +
> > +	guard(write_lock_irqsave)(&vcpu->kvm->mmu_lock);
> > +	for (idx = 0; idx < curr_idx; idx++) {
> > +		u64 gpa;
> > +
> > +		gpa = hdbss_buf[idx];
> > +		if (!(gpa & HDBSS_ENTRY_VALID))
> > +			continue;
> > +
> > +		gpa &= HDBSS_ENTRY_IPA;
> > +		kvm_vcpu_mark_page_dirty(vcpu, gpa >> PAGE_SHIFT);
> > +	}
> 
> Here ^
> 
> > +
> > +	/* reset HDBSS index */
> > +	write_sysreg_s(0, SYS_HDBSSPROD_EL2);
> > +	vcpu->arch.hdbss.next_index = 0;
> > +	isb();
> > +}
> > +
> > +static int kvm_handle_hdbss_fault(struct kvm_vcpu *vcpu)
> > +{
> > +	u64 prod;
> > +	u64 fsc;
> > +
> > +	prod = read_sysreg_s(SYS_HDBSSPROD_EL2);
> > +	fsc = FIELD_GET(HDBSSPROD_EL2_FSC_MASK, prod);
> > +
> > +	switch (fsc) {
> > +	case HDBSSPROD_EL2_FSC_OK:
> > +		/* Buffer full, which is reported as permission fault. */
> > +		kvm_flush_hdbss_buffer(vcpu);
> > +		return 1;
> > +	case HDBSSPROD_EL2_FSC_ExternalAbort:
> > +	case HDBSSPROD_EL2_FSC_GPF:
> > +		return -EFAULT;
> > +	default:
> > +		/* Unknown fault. */
> > +		WARN_ONCE(1,
> > +				"Unexpected HDBSS fault type, FSC: 0x%llx (prod=0x%llx, vcpu=%d)\n",
> > +				fsc, prod, vcpu->vcpu_id);
> > +		return -EFAULT;
> > +	}
> > +}
> > +
> >  /**
> >   * kvm_handle_guest_abort - handles all 2nd stage aborts
> >   * @vcpu:	the VCPU pointer
> > @@ -2071,6 +2138,9 @@ int kvm_handle_guest_abort(struct kvm_vcpu *vcpu)
> > 
> >  	is_iabt = kvm_vcpu_trap_is_iabt(vcpu);
> > 
> > +	if (esr_iss2_is_hdbssf(esr))
> > +		return kvm_handle_hdbss_fault(vcpu);
> > +
> >  	if (esr_fsc_is_translation_fault(esr)) {
> >  		/* Beyond sanitised PARange (which is the IPA limit) */
> >  		if (fault_ipa >= BIT_ULL(get_kvm_ipa_limit())) {
> > diff --git a/arch/arm64/kvm/reset.c b/arch/arm64/kvm/reset.c
> > index 959532422d3a..c03a4b310b53 100644
> > --- a/arch/arm64/kvm/reset.c
> > +++ b/arch/arm64/kvm/reset.c
> > @@ -161,6 +161,9 @@ 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);
> > +
> > +	if (vcpu->kvm->arch.enable_hdbss)
> > +		kvm_arm_vcpu_free_hdbss(vcpu);
> >  }
> > 
> >  static void kvm_vcpu_reset_sve(struct kvm_vcpu *vcpu)
> > --
> > 2.33.0
> >