From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id BC0D7C43458 for ; Tue, 14 Jul 2026 10:19:54 +0000 (UTC) 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-Owner; bh=YaoPdDxNv5fYqLdnbzZNOsRBkTpIAvs6q+yYe/TCu3k=; b=rCVINlwSPUBaAMRM9DhRYrX7is ZskhWH+OoqpUX42cEzERX6QqueCSYa3ldfAeG/Kqaj4Zdd0CvbPgGpMMukmJSqpJZCR+PRWbSSHoa yfOnY46A3a0LGihC2xB2nenOxRPpCrG0/38FVwztnb0okORl+NkZiGl/mZnofqCWLTNvaBiU8RXz/ oz1oV/SnHLUGiQhBE4hGPHqkVc+U7rzp7YScG/2ENHSqbDweTQX9Py6tMKKiYbniPD/dD0gOFULdL XnndjpaSWlsmaOIHV6xBKlVukH8dbRe9+tiC+leYaOg2wFOAyny1f9biEcPrNeCu13R9W5Wu3xzyq +trI8Hzg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wjaEu-0000000BYCz-2uX8; Tue, 14 Jul 2026 10:19:48 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1wjaEr-0000000BYCS-3X7f for linux-arm-kernel@lists.infradead.org; Tue, 14 Jul 2026 10:19:47 +0000 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 28155497; Tue, 14 Jul 2026 03:19:40 -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 0CBD93F7B4; Tue, 14 Jul 2026 03:19:40 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1784024384; bh=YXLhgjk9qd3m9U6QCUpLlON7Ad8A3ET8lcvFZ6qIMes=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=UZUoTvPwGqlDB68l3l4SN2HegcOjxbVpbmWb6j65q6WMuws3Xp5XyfHNrGeIRirFN ojqapaCRtuGNjw31KWxhtYif9PPHWr0AzM/3WiUjgCJFAngyCz4gFSkWKAwHkbIFlX JDX5RXkrsr8o9/77EP/yLoHz/8tqH9LkKhyJNgZM= 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 0/6] Support the FEAT_HDBSS introduced in Armv9.5 Date: Tue, 14 Jul 2026 11:19:38 +0100 Message-ID: X-Mailer: git-send-email 2.55.0 In-Reply-To: <8757dfcf-aae4-4716-9ff1-c3140d4cc63e@huawei.com> References: <20260709104026.2612599-1-zhengtian10@huawei.com> <8757dfcf-aae4-4716-9ff1-c3140d4cc63e@huawei.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.9.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260714_031946_097608_1425BAD2 X-CRM114-Status: GOOD ( 31.60 ) 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 Tue, Jul 14, 2026 at 05:37:58PM +0800, Tian Zheng wrote: > > On 7/13/2026 6:31 PM, Leonardo Bras wrote: > > On Thu, Jul 09, 2026 at 06:40:20PM +0800, Tian Zheng wrote: > > > This series of patches add support to the Hardware Dirty state tracking > > > Structure (HDBSS) feature, which is introduced by the ARM architecture > > > in the DDI0601 (ID121123) version. > > > > > > The HDBSS feature is an extension to the architecture that enhances > > > tracking translation table descriptors' dirty state, identified as > > > FEAT_HDBSS. This feature utilizes hardware assistance to achieve dirty > > > page tracking, aiming to significantly reduce the overhead of scanning > > > for dirty pages. > > > > > > The purpose of this feature is to make the execution overhead of live > > > migration lower to both the guest and the host, compared to existing > > > approaches (write-protect or search stage-2 tables). > > > > > > The required sysreg definitions for FEAT_HDBSS have been merged into > > > arm64 /sysregs: > > > [1/5] arm64/sysreg: Add HDBSS related register information > > > https://git.kernel.org/arm64/c/72f7be0c2e30 > > > > > > > > > After these patches, the kernel automatically enables HDBSS when dirty > > > logging is enabled on any memslot, and disables HDBSS when dirty logging > > > is disabled on all memslots. This series does not support dirty ring > > > mode. > > > > > > Depends-on: "KVM: arm64: Enable eager hugepage splitting if HDBSS is available" > > > https://lore.kernel.org/linux-arm-kernel/20260629111820.1873540-3-leo.bras@arm.com/ > > On this, FYI, there have been some discussion on this: > > https://lore.kernel.org/all/alETGFD2Ogx6N0HB@LeoBrasDK/ > > > > Oliver's suggestion is that we don't automatically enable eager splitting, > > but instead we have different behaviours if the user enables it. > > > > This is still under discussion there, but I think it can be useful reading. > > > Thanks for the pointer — I've read through the discussion between you and > Oliver, and it's very helpful. > > > If I understand correctly, the proposed approach is to support both modes, > with DBM behavior depending > > on the user's eager split setting: I am still waiting for his feedback on that, but I suppose that's what he meant. > > - If users enable eager splitting (chunk_size != 0): use the v4 approach — > DBM is set globally. > > - If users do NOT enable eager splitting (chunk_size == 0, i.e., lazy split > mode): use the v3 approach — DBM > > is set lazily in user_mem_abort on the first write fault. > > > This makes sense to me. It gives users the flexibility to choose. > Yes, agree > > As I mentioned in my patch 6/6 reply (https://lore.kernel.org/all/6e8b23bc-d420-4f5a-a921-5a5d64d84200@huawei.com/), > > I do think the lazy DBM approach is safer overall if the first-fault > overhead is acceptable — it avoids accidentally marking > > special mappings and naturally handles lazy split. > I remember a previous discussion in which was concluded that lazy marking DBM carried some issues. I have to go back on that and check if we can take care of that now. > > I'll keep an eye on the discussion and follow up once a conclusion is > reached. > Thanks! Leo