From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=113.46.200.221 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793282549; b=dPQw4hBkROknYNBB5zA74DnU9vcSCYnqv6+OLR2JJuqroq46I6oHuez6Qj8Fa3zCAv68 TuapTBOtlIyscCtM/eCtxfycHTNSIHF6BtBvBPb9lVVq4hbnjCziWZ1oRrVotSuRRaqBH cAsu0N9EIQfhLpaOOZC1YfuyMeaSqJu8tfGUhaD11pEG3eRZOMhT/4uWRLIi1V3/sp0O7 97vE1EHJvYi5tTnx6uXB0i9ACsEQFTPlz7WgfcY3WGvzGQgOcOyY8S66DUVpxkyRgD7V4 yCogqF5/hHr6xLYq5Gedmpylx2lbFRF14lZNxtULocslgSOKlk/9QkEaoSCqS5W0UeQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793282549; h=DMARC-Filter:dkim-signature:Received:Received:Received:Message-ID: Date:MIME-Version:User-Agent:Subject:To:CC:References:From: In-Reply-To:Content-Type:Content-Transfer-Encoding:X-Originating-IP: X-ClientProxiedBy; bh=46BTS2L17vKUMaAXzO89PlCn4e9X2ecYyyZ5hRRDLBc=; b=WS9Phh0Zw8LWjnC4ojlp9jcodm0sjhRCYzM4FTlOIgbB/DDt8AfBer5c7Sb+cmCJyHgV hbWCbKa0KrTeKjddRi7JYlL/2XF4HKT0mUWN7g6RzM0A9f+p/BnSMamt+EMJMUYdUiBuI wIUGXAYZX8WiMJnqedYH0W8iItjBj4VAIF2lwIJIS0WNqzF8905aA9GNj3IoYxkF5vzzC FnD8CsDKgAsokuYbwviMRBP3F932m1J3xor4KGDqejzduXDvLxxRq6//IJEwnJYaaKMwX ZlVsuI0Nk9B19Wp634g857ibrqGLKVJMqwZmAT14ahNEN3rPDXt0kIACAF4QvlkXfUQ== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; dkim=pass header.d=huawei.com header.i=@huawei.com header.a=rsa-sha256 header.s=dkim header.b=Xxcr1uEv; arc=none smtp.remote-ip=113.46.200.221 Received: from canpmsgout06.his.huawei.com (canpmsgout06.his.huawei.com [113.46.200.221]) (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 C7C28606A336; Thu, 29 Oct 2026 14:02:20 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.4.2 smtp.subspace.kernel.org C7C28606A336 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key, unprotected) header.d=huawei.com header.i=@huawei.com header.a=rsa-sha256 header.s=dkim header.b=Xxcr1uEv dkim-signature: v=1; a=rsa-sha256; d=huawei.com; s=dkim; c=relaxed/relaxed; q=dns/txt; h=From; bh=46BTS2L17vKUMaAXzO89PlCn4e9X2ecYyyZ5hRRDLBc=; b=Xxcr1uEvjyUncblEyLuJ13eru1beYW/GY5GFrPE5ZiK43B4KlA7E+sfVr4qhA3hrvIBxIwDSM 8bbmRWDAsZAUJOf4aPz+Fkbsf8dlBiYZwCbJ4LCU36/fQiGVBZ8WrYc5WWyoqZLYFc7PE1SX2xp egChEha82ieFCMIiUlrLwNo= Received: from mail.maildlp.com (unknown [172.19.163.0]) by canpmsgout06.his.huawei.com (SkyGuard) with ESMTPS id 4jFmBT2KpwzRhQX; Thu, 29 Oct 2026 22:02:09 +0800 (CST) Received: from kwepemr100010.china.huawei.com (unknown [7.202.195.125]) by mail.maildlp.com (Postfix) with ESMTPS id 162B340561; Thu, 29 Oct 2026 22:02:12 +0800 (CST) Received: from [10.67.120.103] (10.67.120.103) by kwepemr100010.china.huawei.com (7.202.195.125) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.2562.45; Thu, 29 Oct 2026 22:02:10 +0800 Message-ID: <009b13c6-f618-43fe-8a56-8519d31b239b@huawei.com> Date: Thu, 29 Oct 2026 22:02:10 +0800 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 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 05/15] KVM: arm64: Harvest stage-2 dirty state into the host folio account To: Leonardo Bras CC: , , , , , , , , , , , , , , , , , , , , , , , , , , , , References: <20260929103655.85107-1-zhengtian10@huawei.com> <20260929103655.85107-6-zhengtian10@huawei.com> From: Tian Zheng In-Reply-To: Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-ClientProxiedBy: kwepems500002.china.huawei.com (7.221.188.17) To kwepemr100010.china.huawei.com (7.202.195.125) On 10/27/2026 2:05 AM, Leonardo Bras wrote: > Hi Tian, > Sorry for the delay in reviewing... > > On Tue, Sep 29, 2026 at 06:36:45PM +0800, Tian Zheng wrote: >> With VTCR_EL2.HD set, hardware promotes writable-clean descriptors to >> writable-dirty without any VM exit, and the only record of the write >> is the S2AP[1] bit in the stage-2 PTE. The write never passed through >> the host stage-1, and no unmap path reads the bit, so the record dies >> with the PTE and reclaim may discard written guest data (silent >> corruption). > > Sorry, I didn't get what you are trying to achieve here. > > On your patch, I see you marking backing pages dirty at wrprotect and unmap > walkers, and it's very confusing to me on why you are doing this. > > IIUC, what you are trying to do is make sure a page does not lose it's > contents in case it is reclaimed by the OS. > > IIUC, when a page is reclaimed, it will unmap the S2 PTE, and save it's > contents to swap, so when the guest read or write to that page will cause > it to fault. > > The page can never be just unmaped, as if there was a single write to it, > the backing pages should be writable, and the contents need to be saved > somewhere. > > So, if I was able to understand your suggestion, we don't need to worry > about this, and this patch wouldn't be necessary. > > Did you hit this issue? > Can you please expand on your issue, if I could not describe it above? > > Thanks! > Leo Hi Leo, Thanks for the review. I haven't hit this in practice, and based on your analysis the patch doesn't look necessary. Reclaim only writes back folios whose PG_dirty flag is set; a clean folio is simply freed. A stage-2 write never touches the host stage-1, so the folio depends entirely on kvm_release_faultin_page() at fault-in to be marked as dirty; without it, the page stays clean from the host's perspective and reclaim drops the data. Your **RFC v1** patch 2 changed that marking from writable to dirty: ``` - kvm_release_faultin_page(kvm, s2vi->page, !!ret, writable); + kvm_release_faultin_page(kvm, s2vi->page, !!ret, dirty); ``` During logging, a read fault maps the page as WC: the folio is not marked PG_dirty in kvm_s2_fault_map() and hardware promotes the PTE to WD on write without any fault, so reclaim frees the page without saving it (the folio was treated as clean). Therefore, **HDBSS v5** patch 2 reverted to marking the folio for any write-permitting mapping: ``` kvm_release_faultin_page(kvm, s2vi->page, !!ret, writable); ``` Together with the v5 scheme, this makes patch 5 unnecessary: - kvm_s2_fault_compute_prot: always set PROT_W when map_writable - stage2_set_prot_attr: PROT_W -> DBM unconditionally --------- On a related note, I'd like to discuss the next version of the DBM scheme, following Oliver's suggestion [1]. Oliver asked for the write bit to be selected by the system capability: FEAT_HAFDBS: PROT_W => DBM, PROT_DIRTY => S2AP[1] !FEAT_HAFDBS: both => S2AP[1] However, our v5 kvm_s2_fault_compute_prot() granted PROT_W unconditionally. With PROT_W => S2AP[1] on !FEAT_HAFDBS, a read fault on a dirty-tracked memslot would install a writable page. Since dirty- logging relies on the write fault to observe the page, we would fail to track it. So we also need to fix kvm_s2_fault_compute_prot(): ``` if (s2vi->map_writable) { if (s2vi->device || !memslot_is_logging(s2fd->memslot) || kvm_is_write_fault(s2fd->vcpu)) { *prot |= KVM_PGTABLE_PROT_W | KVM_PGTABLE_PROT_DIRTY; } else if (kvm_supports_hafdbs()) { *prot |= KVM_PGTABLE_PROT_W; } } ``` For normal RAM on a system without FEAT_HAFDBS: logging + read fault -> PTE read-only logging + write fault -> PTE S2AP[1]=1 [1] https://lore.kernel.org/all/ff218960-c412-4283-aa41-b7af2a7048b8@huawei.com/ Thanks, Tian