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Wed, 15 Jul 2026 17:16:52 +0800 (CST) Received: from [10.67.120.103] (10.67.120.103) by kwepemr100010.china.huawei.com (7.202.195.125) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.36; Wed, 15 Jul 2026 17:16:51 +0800 Message-ID: Date: Wed, 15 Jul 2026 17:16:51 +0800 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 4/6] KVM: arm64: Add HDBSS per-vCPU buffer management To: Leonardo Bras CC: , , , , , , , , , , , , , , , , , , References: <20260709104026.2612599-1-zhengtian10@huawei.com> <20260709104026.2612599-5-zhengtian10@huawei.com> From: Tian Zheng In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 8bit X-Originating-IP: [10.67.120.103] X-ClientProxiedBy: kwepems200002.china.huawei.com (7.221.188.68) To kwepemr100010.china.huawei.com (7.202.195.125) X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260715_021706_684254_ED89189F X-CRM114-Status: GOOD ( 49.41 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org 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 >>>> >>>> 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 >>>> Signed-off-by: Tian Zheng >>>> --- >>>> 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 >>> 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 >>>> +#include >>>> + >>>> +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. 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 Does that work for you? Thanks! Tian