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

From: Leonardo Bras <[email-masked]>
Date: Fri, 27 Mar 2026 15:00:12 +0000

On Fri, Mar 27, 2026 at 03:35:29PM +0800, Tian Zheng wrote:
> 
> On 3/26/2026 2:05 AM, 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)
> 
> 
> Of course! Happy to exchange ideas and learn together.

:)

> 
> 
> > 
> > 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).
> > 
> > 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
> 
> Hi Leo,
> 
> Thanks for the helpful suggestion. I had previously traced the
> hugepage-splitting path
> 
> during live migration and found that when migration starts, enabling dirty
> logging
> 
> triggers the splitting path. I also tested HDBSS with traditional hugepages
> and haven't
> 
> observed any issues yet.
> 
> 
> However, your concern is valid — there may be cases not covered, especially
> when the
> 
> VMM uses transparent hugepages. I'll integrate your patch into the next
> version and
> 
> run some tests.
> 
> 
> For reference, here's the path I traced:
> 
> ```
> 
> - userspace, e.g., QEMU
> 
> kvm_log_start
> +-> kvm_section_update_flags
>     +-> kvm_slot_update_flags
>         |
>         | // For each memory region, QEMU issues a
> KVM_SET_USER_MEMORY_REGION ioctl.
>         | // Before issuing it, flags are updated to include
> KVM_MEM_LOG_DIRTY_PAGES.
>         +-> kvm_mem_flags
>         +-> kvm_set_user_memory_region   // ioctl that enables dirty logging
> on the memslot
> 
> - KVM
> 
> KVM_SET_USER_MEMORY_REGION
> +-> kvm_vm_ioctl_set_memory_region
>     +-> kvm_set_memory_region / __kvm_set_memory_region
>         +-> kvm_set_memslot
>             +-> kvm_commit_memory_region
>                 +-> kvm_arch_commit_memory_region
>                     +-> kvm_mmu_split_memory_region
>                         // Splits Stage-2 hugepages/contiguous mappings into
> 4KB PTEs.

Right, except on a case we have dirty_log_manual_protect and init_set, when 
it returns before splitting pages:

```
if (kvm_dirty_log_manual_protect_and_init_set(kvm))
	return;
```

IIUC, that's desired to avoid holding the lock for a long time while it 
cleans every page in the beginning, and instead do it in a per dirty-page
basis. I guess it may benefit guests with very little dirty pages, as it 
does not have to split/dirty everything at the start. 
(Its a pain for my HACDBS routines, though)

>                         +-> kvm_mmu_split_huge_pages

Other important point here:
You can see in this function it skips splitting if chunk_size == 0.
This value is set by a capability that configures EAGER_SPLIT, meaning 
splitting before the guest have write faults, which is nice as the 
write-fault is faster.

Two points in this capability:
- It's optional, if it's not set, only on-demand splitting (on fault) will 
  happen, and since HDBSS removes the write-fault, we have no splitting
- It can be set to any valid block size, not only 4K, nor PAGE_SIZE, it can
  be set to PMD_SIZE, PUD_SIZE, and so on, which will depend on the 
  PAGE_SIZE the kernel was compiled to.
 
That's only some points to keep in mind :)

		if (kvm_dirty_log_manual_protect_and_init_set(kvm))
			return;

>                             +-> kvm_pgtable_stage2_split
> 
> ```
> 
> Thanks again for the detailed explanation and for sending the patch.
> 

Thank you for the collaboration on this!
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 ^
> 
> Thanks!
> 
> Tian
> 
> 
> > 
> > > +
> > > +	/* 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
> > >