Re: [PATCH v4 5/6] KVM: arm64: Add HDBSS fault handling and buffer flush

[Leonardo Bras <leo.bras@arm.com>]

On Fri, Jul 17, 2026 at 02:51:12PM +0800, Tian Zheng wrote:
> 
> On 7/14/2026 10:19 PM, Leonardo Bras wrote:
> > On Tue, Jul 14, 2026 at 09:27:15PM +0800, Tian Zheng wrote:
> > > On 7/14/2026 6:50 PM, Leonardo Bras wrote:
> > > > On Tue, Jul 14, 2026 at 03:38:39PM +0800, Tian Zheng wrote:
> > > > > On 7/13/2026 10:06 PM, Leonardo Bras wrote:
> > > > > > On Thu, Jul 09, 2026 at 06:40:25PM +0800, Tian Zheng wrote:
> > > > > > > From: eillon <yezhenyu2@huawei.com>
> > > > > > > 
> > > > > > > Add HDBSS fault handling for buffer full, external abort, and general
> > > > > > > protection fault (GPF) events. When the HDBSS buffer becomes full,
> > > > > > > the hardware traps to EL2 with an HDBSSF event, which is handled by
> > > > > > > setting a flush request.
> > > > > > > 
> > > > > > > Add kvm_flush_hdbss_buffer() to consume HDBSS buffer entries and
> > > > > > > propagate dirty information into the userspace-visible dirty bitmap.
> > > > > > > Flush is triggered on vcpu_put, check_vcpu_requests, and
> > > > > > > sync_dirty_log.
> > > > > > > 
> > > > > > > Add esr_iss2_is_hdbssf() helper for HDBSS fault detection in guest
> > > > > > > abort handling.
> > > > > > > 
> > > > > > > Signed-off-by: Eillon <yezhenyu2@huawei.com>
> > > > > > > Signed-off-by: Tian Zheng <zhengtian10@huawei.com>
> > > > > > > ---
> > > > > > >     arch/arm64/include/asm/esr.h           |  5 +++
> > > > > > >     arch/arm64/include/asm/kvm_dirty_bit.h | 11 +++++
> > > > > > >     arch/arm64/include/asm/kvm_host.h      |  1 +
> > > > > > >     arch/arm64/kvm/arm.c                   | 14 ++++++
> > > > > > >     arch/arm64/kvm/dirty_bit.c             | 62 ++++++++++++++++++++++++++
> > > > > > >     arch/arm64/kvm/mmu.c                   |  4 ++
> > > > > > >     6 files changed, 97 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;
> > > > > > This will return a long, which will be casted as bool.
> > > > > > In general, what I see in the kernel is something like:
> > > > > > 
> > > > > > 	return !!(ESR_ELx_ISS2(esr) & ESR_ELx_HDBSSF)
> > > > > ok!
> > > > > 
> > > > > 
> > > > > > > +}
> > > > > > > +
> > > > > > >     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_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h
> > > > > > > index 84b12f0a10af..4b28000e972f 100644
> > > > > > > --- a/arch/arm64/include/asm/kvm_dirty_bit.h
> > > > > > > +++ b/arch/arm64/include/asm/kvm_dirty_bit.h
> > > > > > > @@ -10,7 +10,18 @@
> > > > > > >     #include <asm/kvm_pgtable.h>
> > > > > > >     #include <asm/sysreg.h>
> > > > > > > 
> > > > > > > +/* HDBSS entry field definitions */
> > > > > > > +#define HDBSS_ENTRY_VALID	BIT(0)
> > > > > > > +#define HDBSS_ENTRY_TTWL_SHIFT (1)
> > > > > > > +#define HDBSS_ENTRY_TTWL_MASK (GENMASK(3, 1))
> > > > > > > +#define HDBSS_ENTRY_TTWL(x) \
> > > > > > > +	(((x) << HDBSS_ENTRY_TTWL_SHIFT) & HDBSS_ENTRY_TTWL_MASK)
> > > > > > > +#define HDBSS_ENTRY_TTWL_RESV HDBSS_ENTRY_TTWL(-4)
> > > > > > > +#define HDBSS_ENTRY_IPA	GENMASK_ULL(55, 12)
> > > > > > > +
> > > > > > >     int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu, unsigned int order);
> > > > > > >     void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu);
> > > > > > > +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu);
> > > > > > > +int kvm_handle_hdbss_fault(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 c41ec6d9c45a..cecfb884a64f 100644
> > > > > > > --- a/arch/arm64/include/asm/kvm_host.h
> > > > > > > +++ b/arch/arm64/include/asm/kvm_host.h
> > > > > > > @@ -55,6 +55,7 @@
> > > > > > >     #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)
> > > > > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> > > > > > > index bf6688245d83..566953a4e23a 100644
> > > > > > > --- a/arch/arm64/kvm/arm.c
> > > > > > > +++ b/arch/arm64/kvm/arm.c
> > > > > > > @@ -755,6 +755,9 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > > > > > >     		kvm_vcpu_put_hw_mmu(vcpu);
> > > > > > >     	kvm_arm_vmid_clear_active();
> > > > > > > 
> > > > > > > +	if (vcpu->kvm->arch.enable_hdbss)
> > > > > > > +		kvm_flush_hdbss_buffer(vcpu);
> > > > > > > +
> > > > > > >     	vcpu_clear_on_unsupported_cpu(vcpu);
> > > > > > >     	vcpu->cpu = -1;
> > > > > > >     }
> > > > > > > @@ -1157,6 +1160,9 @@ 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);
> > > > > > >     	}
> > > > > > > 
> > > > > > > @@ -1971,7 +1977,15 @@ 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.
> > > > > > > +	 */
> > > > > > > +	if (!kvm->arch.enable_hdbss)
> > > > > > > +		return;
> > > > > > > 
> > > > > > > +	kvm_make_all_cpus_request(kvm, KVM_REQ_FLUSH_HDBSS | KVM_REQUEST_WAIT);
> > > > > > >     }
> > > > > > > 
> > > > > > >     static int kvm_vm_ioctl_set_device_addr(struct kvm *kvm,
> > > > > > > diff --git a/arch/arm64/kvm/dirty_bit.c b/arch/arm64/kvm/dirty_bit.c
> > > > > > > index 6c7a6ef66b5a..002366337637 100644
> > > > > > > --- a/arch/arm64/kvm/dirty_bit.c
> > > > > > > +++ b/arch/arm64/kvm/dirty_bit.c
> > > > > > > @@ -50,3 +50,65 @@ void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu)
> > > > > > > 
> > > > > > >     	vcpu->arch.hdbss.hdbssbr_el2 = 0;
> > > > > > >     }
> > > > > > > +
> > > > > > > +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu)
> > > > > > > +{
> > > > > > > +	int idx, curr_idx;
> > > > > > > +	u64 *hdbss_buf;
> > > > > > > +	struct kvm *kvm = vcpu->kvm;
> > > > > > > +
> > > > > > > +	if (!kvm->arch.enable_hdbss)
> > > > > > > +		return;
> > > > > > > +
> > > > > > > +	curr_idx = HDBSSPROD_IDX(read_sysreg_s(SYS_HDBSSPROD_EL2));
> > > > > > > +
> > > > > > > +	/* Do nothing if HDBSS buffer is empty or br_el2 is NULL */
> > > > > > > +	if (curr_idx == 0 || vcpu->arch.hdbss.hdbssbr_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);
> > > > > > You mention that it does not support dirty-ring, but above function will
> > > > > > mark the page as dirty in the dirty-ring :/
> > > > > > 
> > > > > In kvm_arm_enable_hdbss_global(), we explicitly check and reject HDBSS
> > > > > enablement if dirty-ring is active:
> > > > > 
> > > > > ```
> > > > > if (kvm->dirty_ring_size)
> > > > >       return 0;
> > > > > ```
> > > > > 
> > > > > So when kvm_flush_hdbss_buffer() runs (which requires enable_hdbss = true),
> > > > > we know for certain that
> > > > > 
> > > > > kvm->dirty_ring_size == 0. Therefore, kvm_vcpu_mark_page_dirty() will always
> > > > > take the dirty_bitmap path,
> > > > > 
> > > > > never the dirty-ring path.
> > > > > 
> > > > > That said, I'll add a comment in kvm_flush_hdbss_buffer() before dirty ring
> > > > > mode is supported, to make this explicit:
> > > > > 
> > > > > ```
> > > > > /*
> > > > >    * HDBSS is mutually exclusive with dirty-ring mode (see
> > > > >    * kvm_arm_enable_hdbss_global()), so kvm_vcpu_mark_page_dirty()
> > > > >    * will update the dirty_bitmap, not the dirty-ring.
> > > > >    */
> > > > > ```
> > > > > 
> > > > Got it :)
> > > > 
> > > > Out of curiosity: which issues have you found on supporting dirty-ring at
> > > > this point?
> > > > 
> > > > Thanks!
> > > > Leo
> > > 
> > > I haven't looked deeply into dirty-ring yet — my main concern is that if
> > > both the dirty
> > > 
> > > ring and HDBSS buffer fill up, the flush path might get blocked or
> > > complicated.
> > > 
> > > For now, I'm planning to match the HDBSS buffer size to the dirty ring size
> > > in v5 and test it.
> > > 
> > > Ideally, the two buffers would be the same size, and the entire dirty
> > > tracking path would use
> > > 
> > Ah, I see the point.
> > 
> > IIRC, when dirty-ring gets full, the kernel returns to userspace with
> > run->exit_reason == KVM_EXIT_DIRTY_RING_FULL, which will warn the VMM to
> > drain the dirty-ring, and that makes space for us draining HDBSS to the
> > dirty-ring again.
> > 
> > The best way to achieve that, as I remember, is to always drain
> > HDBSS as much as possible at guest_exitting. That will make more space to
> > newer HDBSS entries, and we can get userspace to drain the dirty-ring
> > earlier.
> > 
> > I would say to even make HDBSS buffer half (entries) the dirty-ring. Then
> > we can generally fully drain to the dirty-ring and even report ring full
> > if the ring is above a given threshold percentage full.
> > > HDBSS exclusively — no fallback to the legacy dirty bitmap path. If that
> > > works, I think this approach should be fine.
> > > 
> > > Let me know if you have any insights on dirty ring's full-buffer behavior —
> > > that would be helpful.
> > > 
> > > 
> > Will do!
> > 
> > Thanks!
> > Leo
> 
> 
> Thanks for the insights — reusing PML's reservation mechanism makes sense.
> 
> I think we can*keep the HDBSS buffer at 512 entries* (matching

I recommend using a PAGESIZE (512 in 4k, but bigger in other sizes)

> PML_LOG_NR_ENTRIES)
> 
> for now, and *not expose any ioctl for userspace to configure it*. Since the
> kernel
> 
> auto-enables HDBSS, a userspace size knob would be confusing.
> 

Humm, I am in favor of letting the user change it according to it's 
workload. Why would that be confusing?

(Having a default size is useful just for enabling it to work without any 
change in current VMs)

> 
> The reservation logic would be:
> 
> - Implement kvm_cpu_dirty_log_size() on arm64 to return the HDBSS buffer
> entry
> 
> count (512, or 0 if HDBSS is not enabled)

Why a get to log_size? does userspace need to know it's using HDBSS?

Just a set should do, as VMMs can just try to set a value, and if it fails 
(IOCTL does not exist, or invalid value), then it can just go forward. 

> 
> - Reuse kvm_dirty_ring_get_rsvd_entries() to reserve space for one full
> flush:
> 
> KVM_DIRTY_RING_RSVD_ENTRIES + hdbss_entries
> 
> - So soft_limit = dirty_ring_size - (KVM_DIRTY_RING_RSVD_ENTRIES +
> hdbss_entries),
> 
> guaranteeing a full flush always fits
> 
> kvm_flush_hdbss_buffer() at guest_exit pushes via mark_page_dirty_in_slot()
> ->
> 

I think I get the point here: since we can have sw dirtying as well as 
HDBSS tracking, we may get to the point that we don't have enough space to 
flush hdbss -> dirty-ring, right?

> kvm_dirty_ring_push() -> kvm_dirty_ring_soft_full, which triggers
> 
> KVM_EXIT_DIRTY_RING_FULL when needed.
> 
> If soft_limit is hit, KVM_REQ_DIRTY_RING_FULL is set and the next vcpu_run
> exits
> 
> to userspace for QEMU to drain the ring.

So the software dirtying routine would be affected by the soft limit, but 
HADBSS exit would not. It means the dirty-ring would be effectively smaller 
than specified if we are having mostly sw dirtying.

Did I get that right?

> 
> *One more thing: *when userspace sets the dirty ring size via
> 
> KVM_VM_IOCTL_ENABLE_DIRTY_LOG_RING, we already enforce that
> 
> size >= kvm_dirty_ring_get_rsvd_entries(kvm) * sizeof(struct kvm_dirty_gfn)
> 
> or size < PAGE_SIZE. With HDBSS, kvm_dirty_ring_get_rsvd_entries() will
> include the
> 
> HDBSS entries via kvm_cpu_dirty_log_size(), so the same check will
> automatically guarantee
> 
> the ring is large enough to accommodate the HDBSS buffer. No additional
> validation is needed.
> 

An alternative is moving the remaining array entries to the beginning of 
the array, and setting the registers accordingly. The tradeoff is an 
overhead here vs having the full size of the array available later.

Thanks!
Leo