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

From: Leonardo Bras <[email-masked]>
Date: Wed, 4 Mar 2026 12:08:54 +0000

On Wed, Mar 04, 2026 at 11:06:46AM +0800, Tian Zheng wrote:
> 
> Hi Leo,
> 
> 
> On 2/27/2026 10:10 PM, Leonardo Bras wrote:
> > On Fri, Feb 27, 2026 at 06:47:25PM +0800, Tian Zheng wrote:
> > > 
> > > On 2/26/2026 1:46 AM, Leonardo Bras wrote:
> > > > Hi Tian, eillon,
> > > > 
> > > > 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.
> > > > I wonder if it would not be better just to use the feature if available,
> > > > instead of needing to have userspace enabling it.
> > > > 
> > > I agree. If we decide to make HDBSS automatically enabled, then userspace
> > > would no longer need an explicit ioctl to turn it on. In that case, the only
> > > userspace‑visible control we may still need is a parameter to specify the
> > > HDBSS buffer size, with the kernel providing a reasonable default (for
> > > example, 4 KB).
> > > 
> > > Under such a model, the workflow could be simplified to:
> > > 1. HDBSS is automatically enabled during KVM_SET_USER_MEMORY_REGION if the
> > > feature is available.
> > > 2. HDBSS is automatically disabled when the source VM stops.
> > > 
> > I suggest we allocate the buffers and enable HDBSS during the first step of
> > live migration, this way we don't need o have this memory usage during the
> > lifetime of the VM, and we turn on HDBSS only when needed.
> 
> 
> Yes, we also think that enabling this feature and allocating the buffers
> during the
> 
> first step of live migration is the right approach.
> 
> 
> > > > > 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.
> > > > That way we have to actually take a fault for every write you do after
> > > > migration starts.
> > > > 
> > > > What if, instead, we put the DBM bit in every memory faulted as writable
> > > > (or better, due to a write fault), so from the beginning of the VM we
> > > > know if memory is RO, WC (writable-clean) or WD (writable-dirty).
> > > > 
> > > > On top of that, we don't actually have to take faults when migration
> > > > starts, as HDBSS is tracking it all.
> > > > 
> > > That makes sense. Pre‑setting DBM on writable mappings would avoid
> > > taking write faults after migration starts, I will optimize this in the
> > > next version.
> > > 
> > > > > KVM does not scan S2 page tables to consume DBM.
> > > > You mean dirty pages?
> > > > If so, it will actually use dirty-log or dirty-rings to track the dirty
> > > > pages, and not scan S2 on every iteration.
> > > > 
> > > Sorry for the confusion — what I meant is that if I only added the DBM
> > > bit, then relying on DBM for dirty‑page tracking would require scanning
> > > the S2 page tables to find which PTEs have DBM set, and then updating
> > > the dirty bitmap. That would obviously be expensive.
> > > 
> > > However, in the current patch series, DBM is used together with HDBSS.
> > > With HDBSS enabled, the hardware directly tracks the writable‑clean ->
> > > writable‑dirty transitions and push it to HDBSS buffer, so we no longer
> > > need to walk the S2 page tables at all. This is the main reason why
> > > combining HDBSS with DBM provides a meaningful optimization.
> > > 
> > > I will clarify this more clearly in the next version.
> > Got it! Thanks for making it clear!
> > I would just mention that you are transfering the recorded data to the
> > dirty log, then.
> 
> 
> You're welcome! I'm glad the explanation is clear now.
> 
> 
> > > > > 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)
> > > > > +
> > > > Do we actually need the GENMASK above? Could not we use just the
> > > > HDBSSBR_EL2_BADDR_MASK?
> > > > 
> > > > If the base address received in alloc_pages is not properly aligned, we
> > > > might end up writing in some different memory region that we allocated on.
> > > > 
> > > > If you want to actually make sure the mem region is aligned, check just
> > > > after it's allocated, instead of silently masking it at this moment.
> > > > 
> > > > In any case, I wonder if we actually need above defines, as it looks they
> > > > could easily be replaced by what they do.
> > > > 
> > > > 
> > > You're right, I will replace it with HDBSSBR_EL2_BADDR_MASK, and I will
> > > add an explicit check to ensure that the physical address returned by
> > > alloc_pages() is properly aligned.
> > I recommend checking if the used function have any garantees in respect to
> > alignment, so that maybe we may not actually need to check.
> 
> 
> Ok, I will check that and confirm whether the function provides any
> alignment
> 
> guarantees.
> 
>

See below comment
 
> > > I agree that some of them may not be necessary once the alignment is
> > > validated. I will review them and simplify the definitions where
> > > appropriate.
> > Thanks!
> > > > >    /*
> > > > >     * 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);

/**
 * alloc_pages - Allocate pages.
 * @gfp: GFP flags.
 * @order: Power of two of number of pages to allocate.
 *
 * Allocate 1 << @order contiguous pages.  The physical address of the
 * first page is naturally aligned (eg an order-3 allocation will be aligned
 * to a multiple of 8 * PAGE_SIZE bytes).  The NUMA policy of the current
 * process is honoured when in process context.
 *
 * Context: Can be called from any context, providing the appropriate GFP
 * flags are used.
 * Return: The page on success or NULL if allocation fails.
 */

It looks like we are safe from the aspect of alignment, according to the 
documentation on alloc_pages. 

I would rename the variable 'size' here, it could be misleading, even 
though the ioctl docs state that it's the order.

> > > > > +		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;
> > > > If we prefer using a ioctl, I wonder if it would not be better to have a
> > > > arch-generic option that enables hw dirty-bit tracking, and all archs could
> > > > use it to implement their versions when available.
> > > > 
> > > > I guess any VMM would have a much easier time doing it once, than for every
> > > > arch they support.
> > > > 
> > > I think that even if we eventually decide to enable HDBSS by default,
> > > userspace will still need an ioctl to configure the HDBSS buffer size.
> > > So an interface is required anyway.
> > That's a valid argument. But we could as well have those configured based
> > on VM memory size, or other parameter from the VM.
> > Since it could be allocated just during the migration, we may have some
> > flexibility on size.
> > 
> > But sure, we could have a default value and let user (optionally) configure
> > the hdbss percpu bufsize.
> 
> 
> That's a good idea! We can automatically determine an appropriate buffer
> size
> 
> when this feature is enabled during the first step of live migration, and
> then we
> 
> can remove the ioctl interface.
> 
> 
> I will update this in the next version.
> 
> 
> > > Also, it makes sense to expose this as an arch‑generic capability rather
> > > than an ARM‑specific one. I will rename the ioctl to something like
> > > KVM_CAP_HW_DIRTY_STATE_TRACK, and each architecture can implement its
> > > own hardware‑assisted dirty tracking when available.
> > I wonder if we need a new capability for this, at all.
> > Couldn't we only use the feature when available?
> 
> 
> If we decide to enable this feature entirely inside KVM, we could remove
> 
> this interface.
> 

I think that's the best option.

Thanks!
Leo

> 
> > > I will update the interface in the next version.
> > > 
> > Thanks!
> > Leo
> > 
> > > > >    	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();
> > > > > +}
> > > > > +
> > > > >    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_el2p;
> > > > > +	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);
> > > > > +	}
> > > > > +
> > > > > +	/* 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
> > > > > 
> > > > Thx,
> > > > Leo
> > > > 
> 
> Thx,
> 
> Tian
> 
>