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

On 7/17/2026 11:44 PM, Leonardo Bras wrote:
> 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 <[email protected]>
>>>>>>>>
>>>>>>>> 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 <[email protected]>
>>>>>>>> Signed-off-by: Tian Zheng <[email protected]>
>>>>>>>> ---
>>>>>>>>      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?
> 

Yep. And about why not expose an ioctl to userspace, and how the kernel 
should size the HDBSS buffer for both dirty-bitmap and dirty-ring modes 
— please see the discussion below:

https://lore.kernel.org/linux-arm-kernel/[email protected]/

>>
>> *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
> 
>