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 B4CBD1C3F02; Mon, 13 Jan 2025 19:09:31 +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=1736795371; cv=none; b=WAQQrzrS7bsN/crd+9Ob3xQEDqDfrTuOIFULoxbdIVZiCuIeim9eTkEjZFuh+OniqHMIgE7wNq6s74SGqYoT1hn9LBlt5j727NXT+1ecH9dhqw5G1G8E5VkOZ1SDCxYs5iwC+gX6C09YCH/G0x8A7EjYJYtnnpkT1CPQh7C00OI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1736795371; c=relaxed/simple; bh=40tPngi4zfm8S5S8Z7yrIRFnHtEEQr0YVkQSBaEG2nk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qCzPFRPW+j0WFEs8oJG1RxYvqMGe7Jg1mEAYp50GXlW5D2LtQrEOjIQH4qvfYg5SnYHEPDkQKXyKp9nQiDv/88i1E/98RYbIC0yf7q/IMVCZDC1Lhu704GnKoDkMo9G7kS+WnpzK5NMv78jpDkrqF30OOeV3fzvYhztBgZoVmOs= 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 EB76EC4CED6; Mon, 13 Jan 2025 19:09:28 +0000 (UTC) Date: Mon, 13 Jan 2025 19:09:26 +0000 From: Catalin Marinas To: "Aneesh Kumar K.V" Cc: linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, Suzuki K Poulose , Steven Price , Will Deacon , Marc Zyngier , Mark Rutland , Oliver Upton , Joey Gouly , Zenghui Yu , Peter Collingbourne Subject: Re: [PATCH v2 5/7] KVM: arm64: MTE: Use stage-2 NoTagAccess memory attribute if supported Message-ID: References: <20250110110023.2963795-1-aneesh.kumar@kernel.org> <20250110110023.2963795-6-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: On Sat, Jan 11, 2025 at 06:49:55PM +0530, Aneesh Kumar K.V wrote: > Catalin Marinas writes: > > On Fri, Jan 10, 2025 at 04:30:21PM +0530, Aneesh Kumar K.V (Arm) wrote: > >> Currently, the kernel won't start a guest if the MTE feature is enabled > > ... > > >> @@ -2152,7 +2162,8 @@ int kvm_arch_prepare_memory_region(struct kvm *kvm, > >> if (!vma) > >> break; > >> > >> - if (kvm_has_mte(kvm) && !kvm_vma_mte_allowed(vma)) { > >> + if (kvm_has_mte(kvm) && > >> + !kvm_has_mte_perm(kvm) && !kvm_vma_mte_allowed(vma)) { > >> ret = -EINVAL; > >> break; > >> } > > > > I don't think we should change this, or at least not how it's done above > > (Suzuki raised a related issue internally relaxing this for VM_PFNMAP). > > > > For standard memory slots, we want to reject them upfront rather than > > deferring to the fault handler. An example here is file mmap() passed as > > standard RAM to the VM. It's an unnecessary change in behaviour IMHO. > > I'd only relax this for VM_PFNMAP mappings further down in this > > function (and move the VM_PFNMAP check above; see Suzuki's internal > > patch, unless he posted it publicly already). > > But we want to handle memslots backed by pagecache pages for virtio-shm > here (virtiofs dax use case). Ah, I forgot about this use case. So with virtiofs DAX, does a host page cache page (host VMM mmap()) get mapped directly into the guest as a separate memory slot? In this case, the host vma would not have VM_MTE_ALLOWED set. > With MTE_PERM, we can essentially skip the > kvm_vma_mte_allowed(vma) check because we handle all types in the fault > handler. This was pretty much the early behaviour when we added KVM support for MTE, allow !VM_MTE_ALLOWED and trap them later. However, we disallowed VM_SHARED because of some non-trivial race. Commit d89585fbb308 ("KVM: arm64: unify the tests for VMAs in memslots when MTE is enabled") changed this behaviour and the VM_MTE_ALLOWED check happens upfront. A subsequent commit removed the VM_SHARED check. It's a minor ABI change but I'm trying to figure out why we needed this upfront check rather than simply dropping the VM_SHARED check. Adding Peter in case he remembers. I can't see any race if we simply skipped this check altogether, irrespective of FEAT_MTE_PERM. -- Catalin