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

From: Leonardo Bras

Date: Tue Jul 14 2026 - 10:20:08 EST

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@xxxxxxxxxx>
> > > > >
> > > > > 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@xxxxxxxxxx>
> > > > > Signed-off-by: Tian Zheng <zhengtian10@xxxxxxxxxx>
> > > > > ---
> > > > > 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