From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 85CA5606A322 for ; Tue, 27 Oct 2026 18:13:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=jME175ol; dkim=pass (1024-bit key, unprotected) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=MRWCaJvB DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Transfer-Encoding: Content-Type:MIME-Version:References:In-Reply-To:Message-ID:Date:Subject:Cc: To:From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Unsubscribe-Post: List-Owner; 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Tue, 27 Oct 2026 11:13:38 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1793124822; bh=StP32dfIcKd8YphSKpAL+/HMutnlIIEzCaQQmma93NA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=MRWCaJvB6Q28OPTMIyyC6DjI8k5TlohZktMkWptC/He2/3HJxzp5kYwhVFDcW74p3 o33mnJ07QNYP5dUnmQi2BdWSac/cz93NF/qOl3XcNpzMvuUI9ZLP8rq0VqQ5Xx4KuG 8wbM62lsVAv+Qy+Es7f5beZKEbQGV8ixw8A3RHbI= From: Leonardo Bras To: Tian Zheng Cc: Leonardo Bras , maz@kernel.org, oupton@kernel.org, catalin.marinas@arm.com, will@kernel.org, corbet@lwn.net, pbonzini@redhat.com, yuzenghui@huawei.com, wangzhou1@hisilicon.com, yangjinqian1@huawei.com, caijian11@h-partners.com, liuyonglong@huawei.com, tangchengchang@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, fuad.tabba@linux.dev, mark.rutland@arm.com, seanjc@google.com, rdunlap@infradead.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, skhan@linuxfoundation.org Subject: Re: [PATCH v5 08/15] KVM: arm64: Flush the HDBSS buffer on VM exit Date: Tue, 27 Oct 2026 18:13:35 +0000 Message-ID: X-Mailer: git-send-email 2.56.0 In-Reply-To: <20260929103655.85107-9-zhengtian10@huawei.com> References: <20260929103655.85107-1-zhengtian10@huawei.com> <20260929103655.85107-9-zhengtian10@huawei.com> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20261027_111346_677482_6794D199 X-CRM114-Status: GOOD ( 33.92 ) X-Spam-Score: -4.4 (----) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" X-Spam-Status: No, score=-19.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Sep 29, 2026 at 06:36:48PM +0800, Tian Zheng wrote: > From: Eillon > > HDBSS entries accumulate in the per-vCPU buffer while the guest runs, > and must be drained into the dirty bitmap or dirty ring before > userspace can observe them. > > Drain the buffer at a single point: kvm_arch_vcpu_ioctl_run() flushes > it on every VM exit, before the exit reason is handled. Flushing > inside the run loop keeps the dirty-ring feedback timely: when a flush > pushes the ring past its soft limit, kvm_dirty_ring_push() raises > KVM_REQ_DIRTY_RING_SOFT_FULL, which check_vcpu_requests() observes at > the top of the next loop iteration, so the vCPU exits to userspace for > the harvest before re-entering the guest. > > kvm_arch_sync_dirty_log() relies on the exit path to flush: it kicks > vCPUs out of guest mode, and the GET/CLEAR protocol tolerates > concurrent bitmap writes, so the snapshot is complete without extra > synchronization. > > Signed-off-by: Eillon > Signed-off-by: Tian Zheng > --- > arch/arm64/include/asm/kvm_dirty_bit.h | 15 +++++++++ > arch/arm64/kvm/arm.c | 20 ++++++++++++ > arch/arm64/kvm/dirty_bit.c | 45 ++++++++++++++++++++++++++ > 3 files changed, 80 insertions(+) > > diff --git a/arch/arm64/include/asm/kvm_dirty_bit.h b/arch/arm64/include/asm/kvm_dirty_bit.h > index fe703f02626b..d828e6b43fe9 100644 > --- a/arch/arm64/include/asm/kvm_dirty_bit.h > +++ b/arch/arm64/include/asm/kvm_dirty_bit.h > @@ -13,6 +13,9 @@ > #include > #include > > +#define HDBSS_ENTRY_VALID BIT(0) > +#define HDBSS_ENTRY_IPA GENMASK_ULL(55, 12) > + > #define KVM_ARM_HDBSS_DEFAULT_SIZE PAGE_SIZE > #define KVM_ARM_HDBSS_MAX_SIZE SZ_2M > > @@ -22,7 +25,19 @@ static inline u32 kvm_hdbss_buffer_size(struct kvm *kvm) > return kvm->arch.hdbss_buffer_size ?: KVM_ARM_HDBSS_DEFAULT_SIZE; > } > > +static inline bool kvm_hdbss_enabled(struct kvm *kvm) > +{ > + return kvm->arch.mmu.vtcr & VTCR_EL2_HDBSS; > +} > + > +static inline bool vcpu_hdbss_enabled(struct kvm_vcpu *vcpu) > +{ > + return vcpu->arch.hw_mmu && > + (vcpu->arch.hw_mmu->vtcr & VTCR_EL2_HDBSS); > +} > + IIUC, arch.mmu is always the canonical s2_mmu, while hw_mmu can be the nested s2_mmu, if enabled. Was that your intend here? If not, we could use only kvm* version, which can be called with vcpu->kvm. If it was, then I would like to understand the logic here, as IIUC the buffer is accessible only for the non-nested guest, as it's allocated in a per-vcpu structure. (vcpu->arch) > int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu); > void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu); > +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu); > > #endif /* __ARM64_KVM_DIRTY_BIT_H__ */ > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > index 5ea4ac26995e..9d7bdece149a 100644 > --- a/arch/arm64/kvm/arm.c > +++ b/arch/arm64/kvm/arm.c > @@ -1430,6 +1430,14 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu) > > trace_kvm_exit(ret, kvm_vcpu_trap_get_class(vcpu), *vcpu_pc(vcpu)); > > + /* > + * Drain the HDBSS buffer before the exit is handled, so > + * entries pushed to the dirty ring are accounted for by > + * dirty_ring_check_request() on the next iteration. > + */ > + if (vcpu_hdbss_enabled(vcpu)) > + kvm_flush_hdbss_buffer(vcpu); > + > /* Exit types that need handling before we can be preempted */ > handle_exit_early(vcpu, ret); > looks the correct place, haven't looked too much on that, though. > @@ -2017,7 +2025,19 @@ 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) > { > + unsigned long i; > + struct kvm_vcpu *vcpu; > > + if (!kvm_hdbss_enabled(kvm)) > + return; > + > + /* > + * The buffer is drained on every VM exit, so kicking running > + * vCPUs is enough to flush them; the dirty-log GET/CLEAR > + * protocol tolerates bits set concurrently with the snapshot. > + */ > + kvm_for_each_vcpu(i, vcpu, kvm) > + kvm_vcpu_kick(vcpu); > } > > 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 f9aeb9f34ad0..be0d12555c84 100644 > --- a/arch/arm64/kvm/dirty_bit.c > +++ b/arch/arm64/kvm/dirty_bit.c > @@ -13,6 +13,7 @@ > #include > #include > #include > +#include > Out of curiosity, could you please explain you reasoning on selecting srcu as locking here? (Like, what are you protecting against, and why srcu is a good choice) > int kvm_arm_vcpu_alloc_hdbss(struct kvm_vcpu *vcpu) > { > @@ -53,3 +54,47 @@ void kvm_arm_vcpu_free_hdbss(struct kvm_vcpu *vcpu) > vcpu->arch.hdbss.hdbss_pg = NULL; > vcpu->arch.hdbss.hdbssbr_el2 = 0; > } > + > +void kvm_flush_hdbss_buffer(struct kvm_vcpu *vcpu) > +{ > + int idx, curr_idx; > + u64 prod; > + u32 entries; > + u64 *hdbss_buf; > + struct kvm *kvm = vcpu->kvm; > + int srcu_idx; > + > + if (!vcpu_hdbss_enabled(vcpu)) > + return; You test this before calling, is it really necessary to test it again, just after calling? > + > + prod = read_sysreg_s(SYS_HDBSSPROD_EL2); > + curr_idx = HDBSSPROD_IDX(prod); > + > + if (curr_idx == 0 || !vcpu->arch.hdbss.hdbss_pg) > + return; > + > + hdbss_buf = page_address(vcpu->arch.hdbss.hdbss_pg); > + if (!hdbss_buf) > + return; > + > + entries = kvm_hdbss_buffer_size(kvm) / sizeof(u64); > + > + /* kvm_vcpu_mark_page_dirty() resolves the memslot under SRCU. */ > + srcu_idx = srcu_read_lock(&kvm->srcu); > + for (idx = 0; idx < min_t(u32, curr_idx, entries); 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); > + } > + srcu_read_unlock(&kvm->srcu, srcu_idx); > + > + prod &= ~HDBSSPROD_EL2_INDEX_MASK; > + write_sysreg_s(prod, SYS_HDBSSPROD_EL2); > + vcpu->arch.hdbss.hdbssprod_el2 = prod; > + isb(); > +} > -- > 2.43.0 > Thanks! Leo