[PATCH v4 4/6] KVM: arm64: Add HDBSS per-vCPU buffer management
Tian Zheng
zhengtian10 at huawei.com
Thu Jul 16 21:06:01 PDT 2026
On 7/15/2026 10:28 PM, Leonardo Bras wrote:
> On Wed, Jul 15, 2026 at 05:16:51PM +0800, Tian Zheng wrote:
>> On 7/14/2026 6:47 PM, Leonardo Bras wrote:
>>> On Tue, Jul 14, 2026 at 03:15:03PM +0800, Tian Zheng wrote:
>>>> On 7/13/2026 9:39 PM, Leonardo Bras wrote:
>>>>> On Thu, Jul 09, 2026 at 06:40:24PM +0800, Tian Zheng wrote:
>>>>>> From: eillon <yezhenyu2 at huawei.com>
>>>>>>
>>>>>> Add HDBSS (Hardware Dirty Bit State Structure) per-vCPU buffer
>>>>>> management including allocation, freeing, and loading of HDBSS
>>>>>> registers during vCPU load.
>>>>>>
>>>>>> This patch creates the foundational infrastructure:
>>>>>> - struct vcpu_hdbss_state and enable_hdbss/hdbss_order in kvm_arch
>>>>>> - kvm_dirty_bit.h header with alloc/free declarations
>>>>>> - dirty_bit.c with alloc/free helpers
>>>>>> - __load_hdbss() in VHE switch for register loading
>>>>>> - vCPU create/destroy hooks for buffer lifecycle
>>>>>> - sysreg definitions for HDBSS register manipulation
>>>>>> - Makefile update for dirty_bit.o
>>>>>>
>>>>>> Signed-off-by: Eillon <yezhenyu2 at huawei.com>
>>>>>> Signed-off-by: Tian Zheng <zhengtian10 at huawei.com>
>>>>>> ---
>>>>>> arch/arm64/include/asm/kvm_dirty_bit.h | 16 ++++++++
>>>>>> arch/arm64/include/asm/kvm_host.h | 13 +++++++
>>>>>> arch/arm64/include/asm/sysreg.h | 11 ++++++
>>>>>> arch/arm64/kvm/Makefile | 1 +
>>>>>> arch/arm64/kvm/arm.c | 7 ++++
>>>>>> arch/arm64/kvm/dirty_bit.c | 52 ++++++++++++++++++++++++++
>>>>>> arch/arm64/kvm/hyp/vhe/switch.c | 15 ++++++++
>>>>>> arch/arm64/kvm/mmu.c | 1 +
>>>>>> arch/arm64/kvm/reset.c | 4 ++
>>>>>> 9 files changed, 120 insertions(+)
>>>>>> create mode 100644 arch/arm64/include/asm/kvm_dirty_bit.h
>>>>>> create mode 100644 arch/arm64/kvm/dirty_bit.c
>>>>>>
>>>>>> diff --git a/arch/arm64/include/asm/kvm_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h
>>>>>> new file mode 100644
>>>>>> index 000000000000..84b12f0a10af
>>>>>> --- /dev/null
>>>>>> +++ b/arch/arm64/include/asm/kvm_dirty_bit.h
>>>>>> @@ -0,0 +1,16 @@
>>>>>> +/* SPDX-License-Identifier: GPL-2.0-only */
>>>>>> +/*
>>>>>> + * Copyright (C) 2026 ARM Ltd.
>>>>>> + * Author: Leonardo Bras <leo.bras at arm.com>
>>>>> You are adding the file, so the copyright note should be yours, then?
>>>> I originally thought your earlier patchset had already created this file, so
>>>> I
>>>>
>>>> kept your copyright and author info. I'll update it to mine in the next
>>>> version.
>>>>
>>>>
>>>> Thanks for pointing that out!
>>> :)
>>>
>>>>>> + */
>>>>>> +
>>>>>> +#ifndef __ARM64_KVM_DIRTY_BIT_H__
>>>>>> +#define __ARM64_KVM_DIRTY_BIT_H__
>>>>>> +
>>>>>> +#include <asm/kvm_pgtable.h>
>>>>>> +#include <asm/sysreg.h>
>>>>>> +
>>>>>> +int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu, unsigned int order);
>>>>>> +void kvm_arm_vcpu_free_hdbss(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 bae2c4f92ef5..c41ec6d9c45a 100644
>>>>>> --- a/arch/arm64/include/asm/kvm_host.h
>>>>>> +++ b/arch/arm64/include/asm/kvm_host.h
>>>>>> @@ -420,6 +420,10 @@ struct kvm_arch {
>>>>>> */
>>>>>> struct kvm_protected_vm pkvm;
>>>>>>
>>>>>> + /* HDBSS: per-VM dirty tracking state */
>>>>>> + bool enable_hdbss;
>>>>> Is there not a way of checking hdbss without adding this new member on the
>>>>> struct? Would not checking the vcpu_hdbss_state from current vcpu should be
>>>>> enough?
>>>> HDBSS is a VM-wide property, so I currently use a VM-level flag
>>>> (kvm->arch.enable_hdbss)
>>>>
>>>> to track whether it's enabled. Paths like kvm_arch_commit_memory_region()
>>>> and VM teardown
>>>>
>>>> don't have a vCPU context to query, so per-vCPU state wouldn't work there.
>>>>
>>>>
>>>> But you're right, maybe we don't need a value to store it — we could remove
>>>> this flag and instead
>>>>
>>>> check the VTCR_EL2_HDBSS bit directly from kvm->arch.mmu.vtcr:
>>>>
>>>>
>>>> ```
>>>> static inline bool kvm_hdbss_enabled(const struct kvm *kvm)
>>>> {
>>>> return kvm->arch.mmu.vtcr & VTCR_EL2_HDBSS;
>>>> }
>>>>
>>>> if (kvm_hdbss_enabled(kvm))
>>>> ...
>>>> ```
>>> That looks better :)
>>>
>>>>>> + unsigned int hdbss_order;
>>>>> Is that the order for hdbss buffer size?
>>>>> Haven't finished reading the series, but would that be bad if this was also
>>>>> in the per-vcpu state struct, instead?
>>>>>
>>>>> Also, here I suggest that we save a number, instead of the encoding for
>>>>> HDBSSBR_ELS_SZ, and convert it on hdbss setup. It could be either a
>>>>> shift, or size.
>>>>>
>>>>> Reason being that you used this to compare with HDBSS_MAX_ORDER at some
>>>>> point, and there is no guarantee that the encoding will always be in
>>>>> crescent form.
>>>>>
>>>>> Also, it's not clear where you set this value.
>>>> Yes, this is the HDBSS buffer order (2^order bytes per vCPU).
>>>>
>>>> I put it in kvm->arch *to make sure all vCPUs in a VM have the same order*.
>>>>
>>> Right, but do we need to retrieve this value at any context that is not
>>> per-vcpu later? If all allocations happen at the same time, and the vcpus
>>> can get the size of their arrays (even by the register itself), maybe there
>>> is no need to add this member in the kvm_arch structure.
>>>
>>>> I agree we should store a plain order number rather than the raw SZ
>>>> encoding.
>>>>
>>> I would recommend we always save the value in the way its most convenient
>>> to use. In this case, I only recall it to be used as bounds to read the
>>> HDBSS array, so it would be better if we save it as plain size.
>>>
>>>> The assignment should happen in kvm_arm_enable_hdbss_global(), which I
>>>> missed in v4 and will add in v5.
>>>>
>>> Okay
>>>
>>>>>> +
>>>>>> #ifdef CONFIG_PTDUMP_STAGE2_DEBUGFS
>>>>>> /* Nested virtualization info */
>>>>>> struct dentry *debugfs_nv_dentry;
>>>>>> @@ -838,6 +842,12 @@ struct vcpu_reset_state {
>>>>>> bool reset;
>>>>>> };
>>>>>>
>>>>>> +struct vcpu_hdbss_state {
>>>>>> + phys_addr_t base_phys; /* for memory free */
>>>>>> + u64 hdbssbr_el2; /* load directly */
>>>>>> + u64 hdbssprod_el2; /* save directly */
>>>>>> +};
>>>>>> +
>>>>>> struct vncr_tlb;
>>>>>>
>>>>>> struct kvm_vcpu_arch {
>>>>>> @@ -945,6 +955,9 @@ struct kvm_vcpu_arch {
>>>>>>
>>>>>> /* Hyp-readable copy of kvm_vcpu::pid */
>>>>>> pid_t pid;
>>>>>> +
>>>>>> + /* HDBSS registers info */
>>>>>> + struct vcpu_hdbss_state hdbss;/
>>>>>> };
>>>>>>
>>>>>> /*
>>>>>> diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
>>>>>> index 7aa08d59d494..1354a58c3316 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_ORDER HDBSSBR_EL2_SZ_2MB
>>>>>> +
>>>>> See above comment on using the encoding instead of an actual size/shift
>>>>> number.
>>>> You're right — I will fix this in the next version as follows:
>>>>
>>>> First, HDBSS_MAX_ORDER will be defined as a plain order number. In v4 I only
>>>> considered the 4KB page
>>> Maybe consider a plain size number, instead.
>>>
>>>> case and hard-coded it to 9. To make it work for any PAGE_SIZE configuration
>>>> (4KB, 16KB, or 64KB), I'll derive
>>>>
>>>> it from HDBSSBR_EL2_SZ_2MB:
>>>>
>>>> ```
>>>> #define HDBSS_MAX_ORDER (12 + HDBSSBR_EL2_SZ_2MB - PAGE_SHIFT)
>>>> ```
>>>>
>>>> This evaluates to 9 on 4KB pages, and the correct value on 16KB/64KB pages
>>>> as well.
>>> Wait, I don't see what you are trying to achieve here with this.
>>> This order is related to the size of a HDBSS buffer, which is defined
>>> independently of the page size being used. In 2MB example, it does hold
>>> 256K HDBSS entries, disregarding on the page size, IIRC.
>>>
>>>
>>>> Second, I will add two conversion helpers in sysreg.h to bridge between
>>>> kernel semantics (alloc_pages order)
>>>>
>>>> and hardware encoding (HDBSSBR_EL2.SZ):
>>>>
>>>> ```
>>>> /* Convert alloc_pages order to HDBSSBR_EL2.SZ encoding */
>>>> #define hdbss_order_to_sz(order) (PAGE_SHIFT + (order) - 12)
>>>> ```
>>> (would be *_to_shift() no?)
>>>
>>> In any case, see above.
>>>
>>>> These helpers work for any PAGE_SIZE configuration (4KB, 16KB, or 64KB)
>>>> because they use PAGE_SHIFT directly.
>>>>
>>>> Third, I will update the HDBSSBR_EL2 register construction in
>>>> kvm_arm_vcpu_alloc_hdbss()：
>>>>
>>>> ```
>>>> /* before */
>>>> .hdbssbr_el2 = HDBSSBR_EL2(page_to_phys(hdbss_pg), order),
>>>>
>>>> /* after */
>>>> .hdbssbr_el2 = HDBSSBR_EL2(page_to_phys(hdbss_pg),
>>>> hdbss_order_to_sz(order)),
>>>> ```
>>>>
>>> That would be the other way around, right? I mean, here you would want the
>>> order encoded in HDBSSBR_SZ from the page shift, not the other way around.
>>>
>>> In any case, see above.
>>>
>>>
>>>> And I will also update the __free_pages() call to use
>>>> vcpu->kvm->arch.hdbss_order directly, since that field
>>>>
>>>> stores the plain order number and avoids decoding the SZ encoding at free
>>>> time:
>>>>
>>>>
>>>> ```
>>>> /* before */
>>>> __free_pages(hdbss_pg,
>>>> FIELD_GET(HDBSSBR_EL2_SZ_MASK,
>>>> vcpu->arch.hdbss.hdbssbr_el2));
>>>>
>>>> /* after */
>>>> __free_pages(hdbss_pg,
>>>> vcpu->kvm->arch.hdbss_order);
>>>> ```
>>>>
>>> Ah, you use the 'order' to free the buffers as well, ok.
>>> But those are per-vcpu buffers as well, right?
>>>
>>> Again, the HDBSSBR_EL2.SZ AFAIK is not related to page size.
>>>
>>> Thanks!
>>> Leo
>>
>> Got it. I'll store the plain size (in bytes) instead of the order or SZ
>> encoding.
>>
>> One clarification though: I'd like to keep this as a VM-level field
>> (kvm->arch.hdbss_buffer_size) in case we may want to allow userspace to
>> configure the buffer size via ioctl in the future.
> Ah, I see... I agree with that ioctl, as I suggested something similar in
> https://lore.kernel.org/all/alYamq9BL5CTaAX7@LeoBrasDK
>
>> The field would be unsigned long hdbss_buffer_size (in bytes), with helpers
>> to convert to order or SZ encoding when needed:
>> - get_order(size) → for alloc_pages()
>> - ilog2(size) - 12 → for HDBSSBR_EL2.SZ
> nit: ilog2() will round _down_ the buffer size. Take that into account.
>
>> Does that work for you?
> I think it makes sense. Honestly not sure where is the best place to put
> the variable, though. Maybe some of the maintainers can give us some tip.
>
> You plan to make hdbss_buffer_size!=0 just when it's in use, right?
> If that's the case, we can use 'hdbss_buffer_size!=0' as is_hdbss_enabled()
>
> Thanks!
> Leo
Ok. or maybe we can store the number of entries directly, like PML does,
to avoid
conversions. And yes, hdbss_buffer_size != 0 as is_hdbss_enabled() works
for me.
For the variable location — open to maintainers' suggestions.
Thanks,
Tian
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