From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=217.140.110.172 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793979415; b=PVfd9204akcAW7Xeb+uCri9T/dNFye1KlukJdiDCbqOYqznQYUvBa8Ct1q3W8qb534wq yNYK8+onM3XTdlUsAi5Fks7HIKfVvbPQvCo29kR68B73gTpdsZtuWp1BePZt4Cy9hPgeW zgc3mBvMP4kL3gCNS3hrhFcBS1EQIhGSqbbMhr2jIuczq68IyiBX4SYKsb76evAEbtgBb sEy45Bvd86Oos+BNbwH3qsKtSEtUqE5/9TLsl0h1aRE8VozCRV+B+HVMBKXpvsC9zXVT6 Hgop5K4KJk6BQZrdfc5sStES5bcbD0FhWGroVN7JNCrACGw0obQYpK63GIJ2eqQFX0A== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793979415; h=DMARC-Filter:Received:Received:DKIM-Signature:From:To:Cc:Subject: Date:Message-ID:X-Mailer:In-Reply-To:References:MIME-Version: Content-Type:Content-Disposition:Content-Transfer-Encoding; bh=X6oCZ8KMibXCNr1yDQP09IDW1MiX5SNSSfAmHFWcFuM=; b=joZvvL4IkgeBZo1pquQh+QBOJ4ecVGJDLZz/diufGejiXKWlPgR2YODsXPFbKBzzy5a/ 5v0hfokQhSYhdLln3+c87Ou7HYXMuJSqL8KGjVQchC7eLL+53BF4AEjVez+CgIT4eBEzg yl0YZmTWNLAY4rtsW7nSeNNQ2QLhYniJFE9/lLJPfh+xYXhxMa7fB+POz/UnkYMVrHQOn RA+94JeuEknOe7PhZBayHQ7slvA7OpyNVRSRPhjVRcvjo+yxxRPaBlPW6ayvnf87Sb0PB IY8tJQm7hrZh8aRUkBMKraWmm6A60VDTtmgwVptw6Oy5Mvm/3g/Mg5B14YiBZKZQ/rQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=fQxMjKtQ; arc=none smtp.remote-ip=217.140.110.172 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id CA6D7606A33B; Fri, 6 Nov 2026 15:36:54 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org CA6D7606A33B 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, unprotected) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=fQxMjKtQ 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 F22051650; Fri, 6 Nov 2026 07:36:50 -0800 (PST) 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 E638C3F632; Fri, 6 Nov 2026 07:36:48 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1793979414; bh=XhTj1ZqerFVvUKFrBdoT7z+FaNkZhZNX8C/KwXqfi4Y=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=fQxMjKtQP13tykAE+L2nt7gsW2dFPZHgY5a2UhnFNKxU6LLOsJPGDvVyqyrLaJf3g rnqVSo1tCLPaYbCB/JHGdo0YAiCdYF94rO9uZtgxKA4byJK0eOGUkjCjuRQLrU/2Um Qr5BuxuawI84dtwHI4oxWrZiiNGlXsezx2dHHYsI= 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 11/15] KVM: arm64: Reserve dirty ring space for the HDBSS buffer Date: Fri, 6 Nov 2026 15:36:46 +0000 Message-ID: X-Mailer: git-send-email 2.56.0 In-Reply-To: References: <20260929103655.85107-1-zhengtian10@huawei.com> <20260929103655.85107-12-zhengtian10@huawei.com> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline Content-Transfer-Encoding: 8bit On Fri, Nov 06, 2026 at 06:36:04PM +0800, Tian Zheng wrote: > > > On 11/2/2026 10:54 PM, Leonardo Bras wrote: > > On Tue, Sep 29, 2026 at 06:36:51PM +0800, Tian Zheng wrote: > > > The dirty ring reserves room for CPU-side dirty buffers via > > > kvm_cpu_dirty_log_size(). Report the HDBSS buffer entry count > > > through it, so a full buffer flush always fits in the ring. > > > > > > Pin the HDBSS buffer size to PAGE_SIZE when dirty-ring is enabled, > > > via kvm_arch_dirty_ring_size_updated(): a larger buffer only > > > shrinks the soft_limit and forces more frequent userspace drains. > > > > > > Signed-off-by: Tian Zheng > > > --- > > > arch/arm64/kvm/arm.c | 8 ++++++++ > > > arch/arm64/kvm/mmu.c | 8 ++++++++ > > > 2 files changed, 16 insertions(+) > > > > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > > > index 9d7bdece149a..5a7046acb62e 100644 > > > --- a/arch/arm64/kvm/arm.c > > > +++ b/arch/arm64/kvm/arm.c > > > @@ -215,6 +215,14 @@ static int kvm_arm_default_max_vcpus(void) > > > return vgic_present ? kvm_vgic_get_max_vcpus() : KVM_MAX_VCPUS; > > > } > > > > > > +void kvm_arch_dirty_ring_size_updated(struct kvm *kvm) > > > +{ > > > + if (!system_supports_hdbss()) > > > + return; > > > + > > > + kvm->arch.hdbss_buffer_size = KVM_ARM_HDBSS_DEFAULT_SIZE; > > > +} > > > + > > > > On the same subject as a previous patch: > > I don't think this is the best approach, on top of adding a new weak > > function to take care. > > > > Maybe during the dirty-ring allocation, on vcpu creation, we could take > > into account the requested dirty-ring size, and just allocate the HDBSS > > buffer taking that into account. That would make it simpler, don't you > > agree? > > > > Thanks! > > Leo > > Hi Leo, > > The "set once" guard we're discussing comes from the patch that adds the > ioctl, and it isn't really needed. We discussed this earlier [1]: we can > just initialize hdbss_buffer_size to PAGE_SIZE at VM creation and let > the ring reservation use it directly. **That also means > kvm_arch_dirty_ring_size_updated() can be removed.** Thanks for your > suggestion. :) > > As for why PAGE_SIZE is the right value in ring mode: the HDBSS buffer > is flushed on every VM exit, so a larger buffer doesn't reduce the flush > frequency. It only inflates the ring reservation (64 + size/ > sizeof(u64)), which shrinks the soft_limit and forces more frequent > exits to userspace for the ring to be drained. Draining the dirty ring > from QEMU is more expensive than draining the HDBSS buffer from the > kernel, so the smallest buffer gives the best performance. In this mode, > a larger buffer provides no benefit to users. Humm, if properly dimensioned larger buffers would be benefitial for memory-intensive workloads, unless they are guest_exiting for other reasons in the meantime. > > In my tests, I also saw performance degrade when more space is reserved. > Humm, what kind of workloads did you test? Did you have the chance to monitor what were the reasons for VM exiting? I am not saying this is mandatory for the HDBSS implementation, but the whole idea of letting the user choose their own HDBSS buffer size is because different workloads may benefit from different buffer sizes: if a VM writes a lot to new memory, and exits very little for other reasons, then a larger buffer will cause the VM to guest_exit much less often. Does that make sense? Thanks! Leo