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 AEBB9606A31A for ; Mon, 26 Oct 2026 15:02:39 +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=ROQa7PuT; dkim=pass (1024-bit key, unprotected) header.d=arm.com header.i=@arm.com header.a=rsa-sha256 header.s=foss header.b=Fq8fBUGG 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; bh=m9xXitEiI+hi1F9clvRlUMjS8JnuPFDTdYXDE0kyjzc=; b=ROQa7PuTN+T/hc jm/748pAb1Jg6mZXyuVpPmA9MjpJflxaMPnGrk6QbfCoodArM7kdvvjeknmrXnbL7tZ2fVXp4mahV oWTohAzTKJTqqCenncmbz5KLvw7rC15x+8MqgSYMsZJ0AfFjPRrwbE+66fZXoh5GL9gQnFdVehtEO 4kabLl5fozEcJ3pSTOXLbNZPdhPg97ygYZszRMzcqdJ37MRo8nH+4kRyxboTaukEXMys5cG0QrDUt dJv/PU6maQvQ6NSQUXdALw0A563azWzIjFhfPgEpLqToV7E+NPHSc9vtMHvQAmEU5h6zGDFDm4Enn DeNMpD4amS1Z5oV2/c2Q==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLMDD-00000002k0L-1bIf; Mon, 26 Oct 2026 15:02:11 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLMDA-00000002jz5-3Nyy for linux-arm-kernel@lists.infradead.org; Mon, 26 Oct 2026 15:02:10 +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 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-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-20261026_080208_889220_7C1F97B9 X-CRM114-Status: GOOD ( 18.63 ) 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=-9.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,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 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