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=1793026929; b=ozJCk8Rg64iQCSHO+JFo6urUNs5I6PoNHR+s0ys/xarmAkNydTNU8WY85YEpPwo5y82d PeZenJUrbWccwL73ZVemf8FM5tnRgRK5UW9XF9hJKDOloQb17gvxmB/b+p9cY67LwfdBI xbBH2iPcyYWrHryGwVgSqi/7Y1koe16i1SLxV5ouHe5e/0FBvsx6HQBsYZkTqJEzj2vyr jHgflKyIJ/V3P0WZ70PzA0+Ly71wU9KnCf3ZMAdnKUos2C1Ohg0wNluQeiIuwq/X949kf 89DDPJTZ0DWAFwoDz8FeSuCMnnqROaJOj/VUDaUA6ZZh3rISTdpZD6t5+bcmXSiWVYQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793026929; 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=m9xXitEiI+hi1F9clvRlUMjS8JnuPFDTdYXDE0kyjzc=; b=KM04mACNdHYQJbt7WIvaJY9I9A3ffDzkcuDcdHpb0tavuVCML6B0RXPTLdewiPBMMx5D FHefHIA7lsglTfrMAl64uPGuNvpUVlf2mJZGVNAhQCB2yG1CAo5fFrqiFBRfK4eZlRldH sVBFg2BjGmKkzuzC9opH6WQTbirNO+nU/ZHWCFeProPy8FXX7o11/5yhjOdmIw2/sZMdg OtkczIXxaCCGuibp7ubHS+C36LDIVpGPUA/kV+YuZ96OXmG46GBg7Pcao3vYKdctO+ytE 4nkCyuxaWOE02zwfZHZiEep5EahxrR/L0xtkqGHJF1EaGcJxMbOJfxPHDKjM61Mpa+A== 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=Fq8fBUGG; 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 767E1606A31D; Mon, 26 Oct 2026 15:02:08 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org 767E1606A31D 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=Fq8fBUGG 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 AEA1B1655; Mon, 26 Oct 2026 08:02:03 -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 2EA793F7B4; Mon, 26 Oct 2026 08:02:03 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1793026926; bh=v+zgyK0QwsGg1V453RC6kjK16TjZSW8MMUUh5EUtImA=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=Fq8fBUGGhyKB1XXODIVt6BLwqv4o1xkXZa7X2pGzhE3cWxE1Dx6V2Agz2JruU6v8B e6+kt1FW/KX9OCoA7zQKe+M/qRq4dDfLlyRJhVUpUX6qmLxdWvCOV5AohG8CCGKOyh lOSGCTkE5jrlgqOKG5he5sUj7qM29vi1N67m9FTc= From: Leonardo Bras To: Tian Zheng Cc: Leonardo Bras , Oliver Upton , maz@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 01/15] KVM: arm64: pgtables: Change write bit from S2AP_W to DBM Date: Mon, 26 Oct 2026 15:02:00 +0000 Message-ID: X-Mailer: git-send-email 2.56.0 In-Reply-To: <8ca88576-651a-4b7f-91ff-2cb2359aa79e@huawei.com> References: <20260929103655.85107-1-zhengtian10@huawei.com> <20260929103655.85107-2-zhengtian10@huawei.com> <8ca88576-651a-4b7f-91ff-2cb2359aa79e@huawei.com> X-Mailing-List: linux-doc@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 Wed, Sep 30, 2026 at 10:44:26AM +0800, Tian Zheng wrote: > On 9/30/2026 8:25 AM, Oliver Upton wrote: > > Hmm... I'd actually like to see this structured where the "write" bit > > could either be S2AP_W or DBM depending on if the system supports > > FEAT_HAFDBS. Look at how HVHE is handled for selecting the right AP > > encoding for the hyp stage-1 page tables. > > > > Hi Oliver, > > Thanks for the suggestion. I missed the case where FEAT_HAFDBS is not > supported. Systems without the feature should keep the original > encoding, with S2AP[1] as the write permission bit. I'll follow the HVHE > pattern to select between the two encodings in the next version. > > Thanks! > Tian > Hi Oliver, Tian, After some digging, I think it would be better if we kept using S2AP[1] as a writable bit, while DBM can be treated as "Can promote a non-writable page to writable if HAFDBS=on". I know it's not that intuitive, as the Arm ARM even introduced the concept of Writable-clean an Writable-dirty, but I think it will be the best way of referring to what was intended of the bit. I wrote down what I mean by that: https://lore.kernel.org/all/20261026144413.441566-1-leo.bras@arm.com/ tl;dr: - DBM + S2AP[1] is set on writable entries (KVM_PGTABLE_PROT_W) - Writable detection is made only on S2AP[1], so no changes - Wrprotect sets S2AP[1]=0 from the PTE, and also DBM=0 if it's a block On dirty-tracking: - If HAFDBS=0 : no functional change, DBM bit there but unused - If HDBSS=1 - Pages: writable-clean promoted to writable-dirty on write, HDBSS does the tracking - Blocks: faults on write, splits in pages, pages get DBM=1. (argh, I think we need a PROT_DBM for that) Does that make sense? Thanks! Leo