From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 786C42586D1; Mon, 24 Feb 2025 11:05:38 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740395138; cv=none; b=rEF19az24OjawYTVHDK6yQbiBe8/UREGk3467blb7L3lCzjjEJrD9iR/gy0IJys6AczvmMq735CSSjdVnM4yjksh0eFjLO+8yXL4zopN5aKlWxV9gnFK6QHhBwYaeDGJ7wV5ptox2bQV7+b8BExravTffEkNeVNhpo7u3QynlmI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740395138; c=relaxed/simple; bh=Psyn/KQl59WzsnG7eUbev1bYnOJ/uewhEhWQ4K6vZMM=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=AKoiAgPyBmk05queSN4piBY7wvS3wzuaeUYUU3kjc+xo73ZJC3fQYP6TAA7xQvDnoUUp4gTKTyqMOSXxMePK7OGloA/wTZXTnvMRkDIqmL8aaXEMLyvMUKVKztk7c4Eh7jP7ATZg1tOj0zXzpIV8mOYMA+sVQsoa2WNr7V6+sbQ= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 Received: by smtp.kernel.org (Postfix) with ESMTPSA id 551EBC4CEE6; Mon, 24 Feb 2025 11:05:36 +0000 (UTC) Date: Mon, 24 Feb 2025 11:05:33 +0000 From: Catalin Marinas To: "Aneesh Kumar K.V (Arm)" Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, Marc Zyngier , Oliver Upton , Joey Gouly , Zenghui Yu , Will Deacon , Suzuki K Poulose , Steven Price , Peter Collingbourne Subject: Re: [PATCH] KVM: arm64: Drop mte_allowed check during memslot creation Message-ID: References: <20250224093938.3934386-1-aneesh.kumar@kernel.org> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20250224093938.3934386-1-aneesh.kumar@kernel.org> On Mon, Feb 24, 2025 at 03:09:38PM +0530, Aneesh Kumar K.V (Arm) wrote: > Before commit d89585fbb308 ("KVM: arm64: unify the tests for VMAs in > memslots when MTE is enabled"), kvm_arch_prepare_memory_region() only > rejected a memory slot if VM_SHARED was set. This commit unified the > checking with user_mem_abort(), with slots being rejected if either > VM_MTE_ALLOWED is not set or VM_SHARED set. A subsequent commit > c911f0d46879 ("KVM: arm64: permit all VM_MTE_ALLOWED mappings with MTE > enabled") dropped the VM_SHARED check, so we ended up with memory slots > being rejected if VM_MTE_ALLOWED is not set. This wasn't the case before > the commit d89585fbb308. The rejection of the memory slot with VM_SHARED > set was done to avoid a race condition with the test/set of the > PG_mte_tagged flag. Before Commit d77e59a8fccd ("arm64: mte: Lock a page > for MTE tag initialization") the kernel avoided allowing MTE with shared > pages, thereby preventing two tasks sharing a page from setting up the > PG_mte_tagged flag racily. > > Commit d77e59a8fccd ("arm64: mte: Lock a page for MTE tag > initialization") further updated the locking so that the kernel > allows VM_SHARED mapping with MTE. With this commit, we can enable > memslot creation with VM_SHARED VMA mapping. > > This patch results in a minor tweak to the ABI. We now allow creating > memslots that don't have the VM_MTE_ALLOWED flag set. As I commented here: https://lore.kernel.org/r/Z4e04P1bQlFBDHo7@arm.com I'm fine with the change, we basically go back to the original ABI prior to relaxing this for VM_SHARED. > If the guest uses > such a memslot with Allocation Tags, the kernel will generate -EFAULT. > ie, instead of failing early, we now fail later during KVM_RUN. Nit: more like the kernel "will return -EFAULT" to the VMM rather than "generate". > This change is needed because, without it, users are not able to use MTE > with VFIO passthrough (currently the mapping is either Device or > NonCacheable for which tag access check is not applied.), as shown > below (kvmtool VMM). Another nit: "users are not able to user VFIO passthrough when MTE is enabled". At a first read, the above sounded to me like one wants to enable MTE for VFIO passthrough mappings. > [ 617.921030] vfio-pci 0000:01:00.0: resetting > [ 618.024719] vfio-pci 0000:01:00.0: reset done > Error: 0000:01:00.0: failed to register region with KVM > Warning: [0abc:aced] Error activating emulation for BAR 0 > Error: 0000:01:00.0: failed to configure regions > Warning: Failed init: vfio__init > > Fatal: Initialisation failed > > Signed-off-by: Aneesh Kumar K.V (Arm) > --- > arch/arm64/kvm/mmu.c | 5 ----- > 1 file changed, 5 deletions(-) > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > index ef53af6df6de..1f1b5aa43d2d 100644 > --- a/arch/arm64/kvm/mmu.c > +++ b/arch/arm64/kvm/mmu.c > @@ -2178,11 +2178,6 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, > if (!vma) > break; > > - if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) { > - ret = -EINVAL; > - break; > - } > - > if (vma->vm_flags & VM_PFNMAP) { > /* IO region dirty page logging not allowed */ > if (new->flags & KVM_MEM_LOG_DIRTY_PAGES) { Reviewed-by: Catalin Marinas