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 9F55D606A33C for ; Fri, 30 Oct 2026 16:52:42 +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=EGHsoIbx; dkim=pass (1024-bit key, unprotected) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=vsLUxmt6 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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Fri, 30 Oct 2026 09:52:27 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1793379152; bh=7VMMf7kMCadyG94rG4Zx25wwUW0u14+sRWOtUecg2H8=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=vsLUxmt6b/5m7BzFV7Hd8fIIx1EsthiLTP6rN+Ce6YaebNT/TyYfDFqazq9LqUwgB 9RphgrRu+PvjSC9PSvb4hysq0nLDGl2+UBl5il7GrIuZiENqtwODfbcpLMWOpptqBa 6Mh5JBUfaxOKWika3tkdWGger6AD196OTOfbuvSA= 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: Fri, 30 Oct 2026 16:52:08 +0000 Message-ID: X-Mailer: git-send-email 2.56.0 In-Reply-To: <5bbd0d3b-1026-43f3-bc45-245472603144@huawei.com> References: <20260929103655.85107-1-zhengtian10@huawei.com> <20260929103655.85107-9-zhengtian10@huawei.com> <5bbd0d3b-1026-43f3-bc45-245472603144@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-20261030_095234_188502_9E475318 X-CRM114-Status: GOOD ( 38.63 ) 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=-18.8 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,T_SPF_TEMPERROR,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 Fri, Oct 30, 2026 at 12:57:06PM +0800, Tian Zheng wrote: > > > On 10/28/2026 2:13 AM, Leonardo Bras wrote: > > 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) > > > > Hi leo, > > Yes, that was the intent. The shadow MMU never carries VTCR_EL2.HDBSS, > so the flush is naturally skipped when running nested. Ah, right. If we always test the kvm one, we may enable it for the nested guest, which is undesired. [...] > > > > > > @@ -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; > > > + And then here you check for the L1 VM because during migration the host could have dirtied memory, and you want to track that as well. Makes sense > > > + /* > > > + * 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) > > > > kvm_vcpu_mark_page_dirty() resolves the memslot under SRCU > (kvm_vcpu_gfn_to_memslot), and the flush loop calls it for each entry, > so the whole loop needs the read lock held. Oh, right, it's just about getting the memslot... now I see it :) Thanks! Leo