From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 3637B34CFC7; Fri, 17 Jul 2026 15:53:20 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784303602; cv=none; b=FkanjOscXwL0ObFt1SWYk/cQbXTV979jfSm7T79u/55itPAynNhUJrJnlVSUnrl+2KY0ZoMAmsoKWoj26HgQUbI3V7/sOh/+rZJ3r6nIAQBV7NSCG5x6Bl8N8u1Jr3DfRdBHCS5zGGev0vJBZ2TvlktmSJ+LNjNab8cxjAWmyb8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1784303602; c=relaxed/simple; bh=uVZAKCcUBebHyrj+iMVcklRAeeWaNU6Ist3Y6DrLmyw=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version:Content-Type:Content-Disposition; b=NM1FB1Ze7z10l/dWB9LL86wTFf3z/LynxrBWizuOUxh0E+Ul2UmJgIaytuFQLre++6Rda4mx0KaNV4Qw58147nBv1k4irgame7M5vIXoalUjOf1PlLpl8c5TyzbQh4bK0x8alGYzNkWGb7MTSBIizRJPaOVY2iCfSmti47kQvlM= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=liXge0Wq; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="liXge0Wq" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id 3E32D1476; Fri, 17 Jul 2026 08:53:15 -0700 (PDT) Received: from LeoBrasDK.cambridge.arm.com (LeoBrasDK.cambridge.arm.com [10.2.212.21]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 985C93F905; Fri, 17 Jul 2026 08:53:16 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1784303599; bh=uVZAKCcUBebHyrj+iMVcklRAeeWaNU6Ist3Y6DrLmyw=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=liXge0WqZPzKrYesSMI5a8ww23YRny+4IsizmZswUzHvmQgzjEktRp3ZBybILOTMW 1kqYFT7tF9QM6M/upepdMIxySJTISeUB8tiUoRKlGpL9K5ano5nBw1n+tez50grNvW q1rLGG+d0QtM6izGuH4uo3rn+/OwhgyhI5zJdVV4= From: Leonardo Bras To: Tian Zheng Cc: Leonardo Bras , maz@kernel.org, oupton@kernel.org, catalin.marinas@arm.com, will@kernel.org, yuzenghui@huawei.com, wangzhou1@hisilicon.com, yangjinqian1@huawei.com, caijian11@h-partners.com, liuyonglong@huawei.com, yezhenyu2@huawei.com, yubihong@huawei.com, linuxarm@huawei.com, joey.gouly@arm.com, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, seiden@linux.ibm.com, suzuki.poulose@arm.com Subject: Re: [PATCH v4 6/6] KVM: arm64: Add auto HDBSS enable/disable on dirty logging change Date: Fri, 17 Jul 2026 16:53:14 +0100 Message-ID: X-Mailer: git-send-email 2.55.0 In-Reply-To: References: <20260709104026.2612599-1-zhengtian10@huawei.com> <20260709104026.2612599-7-zhengtian10@huawei.com> Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit On Thu, Jul 16, 2026 at 03:15:38PM +0800, Tian Zheng wrote: > > On 7/13/2026 10:50 PM, Leonardo Bras wrote: > > On Thu, Jul 09, 2026 at 06:40:26PM +0800, Tian Zheng wrote: > > > From: eillon > > > > > > HDBSS buffers store per-page dirty state after the stage-2 page tables > > > have been split down to page granularity (chunk_size == PAGE_SIZE). > > chunk_size != PAGE_SIZE now, but that should change as well :) > > > > > When chunk_size == 0 the kernel may lazily skip splitting block mappings, > > > leaving the page table coarser than what HDBSS expects. Therefore, > > > enabling HDBSS requires disabling lazy split so that all block mappings > > > are eagerly broken down before the buffer starts recording. > > (See cover letter reply) > > > > > Add VM-level HDBSS enable/disable support. When dirty logging is > > > enabled on any memslot, HDBSS is automatically enabled. When dirty > > > logging is disabled on all memslots, HDBSS is automatically disabled. > > > > > > This includes: > > > - kvm_arm_enable_hdbss_global() to enable HDBSS for all vCPUs > > > - kvm_arm_disable_hdbss_global() to disable and free HDBSS buffers > > > - kvm_arm_hdbss_on_dirty_logging_change() for auto enable/disable > > > - kvm_arch_destroy_vm() cleanup path > > > - kvm_arch_commit_memory_region() integration > > > > > > Signed-off-by: Eillon > > > Signed-off-by: Tian Zheng > > > --- > > > arch/arm64/include/asm/kvm_dirty_bit.h | 2 + > > > arch/arm64/kvm/arm.c | 8 ++ > > > arch/arm64/kvm/dirty_bit.c | 105 +++++++++++++++++++++++++ > > > arch/arm64/kvm/mmu.c | 3 + > > > 4 files changed, 118 insertions(+) > > > > > > diff --git a/arch/arm64/include/asm/kvm_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h > > > index 4b28000e972f..a4cda8cdab24 100644 > > > --- a/arch/arm64/include/asm/kvm_dirty_bit.h > > > +++ b/arch/arm64/include/asm/kvm_dirty_bit.h > > > @@ -23,5 +23,7 @@ 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); > > > +void kvm_arm_hdbss_on_dirty_logging_change(struct kvm *kvm, int nr_memslots_logging); > > > +void kvm_arm_disable_hdbss_global(struct kvm *kvm); > > > > > > #endif /* __ARM64_KVM_DIRTY_BIT_H__ */ > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > > > index 566953a4e23a..536d94799ba8 100644 > > > --- a/arch/arm64/kvm/arm.c > > > +++ b/arch/arm64/kvm/arm.c > > > @@ -317,6 +317,14 @@ void kvm_arch_destroy_vm(struct kvm *kvm) > > > if (is_protected_kvm_enabled()) > > > pkvm_destroy_hyp_vm(kvm); > > > > > > + /* > > > + * Userspace may destroy the VM without disabling dirty logging, > > > + * so the auto-disable path is never reached. Force disable HDBSS > > > + * here to ensure vCPU buffers are freed and prevent memory leaks. > > > + */ > > > + if (kvm->arch.enable_hdbss) > > > + kvm_arm_disable_hdbss_global(kvm); > > > + > > > kvm_uninit_stage2_mmu(kvm); > > > kvm_destroy_mpidr_data(kvm); > > > > > > diff --git a/arch/arm64/kvm/dirty_bit.c b/arch/arm64/kvm/dirty_bit.c > > > index 002366337637..c5bf866c23ef 100644 > > > --- a/arch/arm64/kvm/dirty_bit.c > > > +++ b/arch/arm64/kvm/dirty_bit.c > > > @@ -112,3 +112,108 @@ int kvm_handle_hdbss_fault(struct kvm_vcpu *vcpu) > > > return -EFAULT; > > > } > > > } > > > + > > > +static unsigned int hdbss_auto_select_order(struct kvm *kvm) > > > +{ > > > + unsigned long npages = 0; > > > + struct kvm_memory_slot *memslot; > > > + int bkt; > > > + > > > + kvm_for_each_memslot(memslot, bkt, kvm_memslots(kvm)) > > > + npages += memslot->npages; > > > + > > > + if (npages <= 16384) > > > + return 0; > > > + else if (npages <= 262144) > > > + return 3; > > > + else if (npages <= 4194304) > > > + return 6; > > > + else > > > + return 9; > > > +} > > IIUC you are counting the amount of pages the VM has, and based on that > > allocating a size for the HDBSS buffer. > > > > A few notes here: > > - It's not really nice to use magic numbers around like this. If you > > actually want to use it, then use stuff like SZ_16K, SZ_256K, SZ_4M and > > so on. > > - You are returning magic numbers as well, why is it 0, 3, 6, or 9 here? > > It only makes sense if the person is reading HDBSSBR_EL2 documentation, > > which should not be necessary at this point. That's one reason I > > recommended to using sizes. If that was really the best way to use it, > > I would recommend using the defines that we get from sysreg, and you > > actually used before to set the maximum order on a previous patch. > > - Also, if you can return only valid values here, why do you check against > > the maximum value in that previous patch? > > - Also, are you using some undisclosed rule here? On 'order 0' the > > meanining is 4KB, which translate to 512 HDBSS entries. Why are you using > > it for any value under 16K? Same for 3-32KB-4kEntries you use for under > > 256K pages (and so on). If you are assuming a logical rule such as > > 'N pages would be ok with N/32 entries' it has to be described here at > > least. > > - Not sure VM size is the best way of doing that, since it will depend > > more on the dirtying rate than the actual size, and most VMs would just > > use the biggest size (4M x 4K pages is just 16GB). For instance with > > dirty_ring we can use the dirty_ring.size as a better option. > > (I know this is a hard one to estimate when using dirty-bitmap, though) > > > > > + > > > +/* > > > + * Enable HDBSS for all vCPUs in the VM. > > > + * > > > + * Called from kvm_arm_hdbss_on_dirty_logging_change() which is invoked > > > + * by kvm_arch_commit_memory_region() under kvm->slots_lock. > > > + * > > > + * If buffer allocation fails, HDBSS remains disabled and dirty tracking > > > + * falls back to the traditional software-based approach (PTE write-protect > > > + * + software dirty marking). This does not affect correctness; dirty > > > + * logging remains functional without HDBSS. > > > + */ > > > +static int kvm_arm_enable_hdbss_global(struct kvm *kvm) > > > +{ > > > + int err; > > > + unsigned long i; > > > + unsigned int order; > > > + struct kvm_vcpu *vcpu; > > > + > > > + if (!system_supports_hdbss()) > > > + return 0; > > > + > > > + if (kvm->dirty_ring_size) /* Don't support HDBSS in dirty ring mode */ > > > + return 0; > > > + > > > + if (kvm->arch.enable_hdbss) /* Already On */ > > > + return 0; > > > + > > > + /* Turn it on */ > > > + order = hdbss_auto_select_order(kvm); > > > + kvm_for_each_vcpu(i, vcpu, kvm) { > > > + err = kvm_arm_vcpu_alloc_hdbss(vcpu, order); > > > + if (err) > > > + goto error_alloc; > > > + } > > > + > > > + kvm->arch.enable_hdbss = true; > > > + kvm->arch.mmu.vtcr |= VTCR_EL2_HD | VTCR_EL2_HDBSS | VTCR_EL2_HA; > > > + > > > + /* > > > + * We should kick vcpus out of guest mode here to load new > > > + * vtcr value to vtcr_el2 register when re-enter guest mode. > > > + */ > > > + kvm_for_each_vcpu(i, vcpu, kvm) > > > + kvm_vcpu_kick(vcpu); > > > + > > > + return 0; > > > + > > > +error_alloc: > > > + kvm_for_each_vcpu(i, vcpu, kvm) > > > + if (vcpu->arch.hdbss.base_phys) > > > + kvm_arm_vcpu_free_hdbss(vcpu); > > > + > > > + pr_warn_once("kvm: failed to allocate HDBSS buffers (order=%u), " > > > + "falling back to software dirty tracking\n", order); > > > + return -ENOMEM; > > > +} > > > + > > > +void kvm_arm_disable_hdbss_global(struct kvm *kvm) > > > +{ > > > + unsigned long i; > > > + struct kvm_vcpu *vcpu; > > > + > > > + if (!kvm->arch.enable_hdbss) /* Already Off */ > > > + return; > > > + > > > + /* Turn it off */ > > > + kvm->arch.mmu.vtcr &= ~(VTCR_EL2_HD | VTCR_EL2_HDBSS | VTCR_EL2_HA); > > > + > > > + kvm_for_each_vcpu(i, vcpu, kvm) > > > + kvm_arm_vcpu_free_hdbss(vcpu); > > > + > > > + kvm->arch.enable_hdbss = false; > > > +} > > > + > > Okay, say the user requested it to be disabled, you change the global vtcr, > > then free the hdbss on every vcpu. > > > > But the vcpus are still running, and since they will only disable this when > > they go out of the guest, then in again, HDBSS will still be running, > > right? > > > > If some page gets dirty in the between, would not the HDBSS try to write to > > the already loaded buffer adress, and write to memory that have already > > been freed here? > > > > > > > +void kvm_arm_hdbss_on_dirty_logging_change(struct kvm *kvm, int nr_memslots_logging) > > > +{ > > > + /* > > > + * Called from kvm_arch_commit_memory_region() under kvm->slots_lock. > > > + * All state transitions are serialized by slots_lock. > > > + */ > > > + if (nr_memslots_logging > 0 && !kvm->arch.enable_hdbss) > > > + kvm_arm_enable_hdbss_global(kvm); > > > + else if (nr_memslots_logging == 0 && kvm->arch.enable_hdbss) > > > + kvm_arm_disable_hdbss_global(kvm); > > > +} > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > > index 949fb895add6..484f48dae000 100644 > > > --- a/arch/arm64/kvm/mmu.c > > > +++ b/arch/arm64/kvm/mmu.c > > > @@ -2588,6 +2588,9 @@ void kvm_arch_commit_memory_region(struct kvm *kvm, > > > { > > > bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES; > > > > > > + kvm_arm_hdbss_on_dirty_logging_change(kvm, > > > + atomic_read(&kvm->nr_memslots_dirty_logging)); > > > + > > > /* > > > * At this point memslot has been committed and there is an > > > * allocated dirty_bitmap[], dirty pages will be tracked while the > > > -- > > > 2.33.0 > > > > > Okay, reading the above I remembered something really complicated: > > We can't really enable HDBSS partially if we start with DBM set for all > > pages. Once we enable HDBSS wit will track changes for all memslots. > > > > The only way to enable it partially would be to set DBM during the > > dirty-bit tracking, which I recall being complicated for some reasons. > > > > Well, we have to think about the overall strategy before a next version. > > > > Thanks! > > Leo > > > Hi, Leo > > > One thing just clicked for me: we don't actually need per-memslot DBM > control. Yeah, I mentioned it here: https://lore.kernel.org/linux-arm-kernel/alZI1rwmeYJHUFut@LeoBrasDK/ > > > > PML on x86 works the same way — D bit set on all writable SPTEs > unconditionally, > > buffer records everything. The filtering happens in software at flush time: > > vmx_flush_pml_buffer() -> kvm_vcpu_mark_page_dirty() -> > mark_page_dirty_in_slot(), > > where kvm_slot_dirty_track_enabled() discards non-tracked memslots. > > > Our HDBSS does exactly the same: global DBM, buffer records all writes, > > kvm_flush_hdbss_buffer() -> mark_page_dirty_in_slot() filters at flush time. > > So I'm questioning the premise — why should HDBSS be held to a different > standard > > than PML? If overhead becomes a real problem, we can revisit per-memslot > later. > > For v5, I think global DBM is fine. > > So, I think we just need to focus on how to correctly handle DBM for huge > pages. > >