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 ABF6D21C191; Wed, 26 Feb 2025 15:58:53 +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=1740585533; cv=none; b=lEuDsPwu/P1JZji/QKiPNT/5IujAN0xiMbFVNZcN/UbQQxmghOBaJnxCsqLNAAd6CdId2djBE+utEEq48kRxbRdyYOv+ANcZrdf1jr1oTecUgON81uqyiPW7geuhYx56ocjErPASK8pJDZETIcS0dFHYe/vMMhonSH2a3lAxZr8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1740585533; c=relaxed/simple; bh=Hk22wqSp4Sd+L0X28JuoHUj1gIOiWKF0tEycajqDzIk=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=qYxszSEXg8SvS2Rl5ld8fEAwx6f54YvCfqzYhgMStX8ut0GA1bt6wBkLmivBtppv1c0S+JQgR66KQTLONGvr4Z2xdJGWuitmEHzyS9uHm7zMF21oi9KhbP58gJi3l1gXQ5gUqrHl+e8pnricCZmBu4KxPIzxZDMTGcGvhwaSLo0= 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 22248C4CED6; Wed, 26 Feb 2025 15:58:50 +0000 (UTC) Date: Wed, 26 Feb 2025 15:58:48 +0000 From: Catalin Marinas To: "Aneesh Kumar K.V" Cc: Marc Zyngier , linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, 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> <86ldtvr0nl.wl-maz@kernel.org> <86jz9fqtbk.wl-maz@kernel.org> <86ikozqmsl.wl-maz@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=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Feb 26, 2025 at 03:28:26PM +0530, Aneesh Kumar K.V wrote: > Marc Zyngier writes: > > On Mon, 24 Feb 2025 16:44:06 +0000, > > Aneesh Kumar K.V wrote: > >> >> On Mon, Feb 24, 2025 at 12:24:14PM +0000, Marc Zyngier wrote: > >> >> > > On Mon, Feb 24, 2025 at 03:09:38PM +0530, Aneesh Kumar K.V (Arm) wrote: > >> >> > > > 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). [...] > >> >> > My other concern is that this gives pretty poor consistency to the > >> >> > guest, which cannot know what can be tagged and what cannot, and > >> >> > breaks a guarantee that the guest should be able to rely on. [...] > >> What if we trigger a memory fault exit with the TAGACCESS flag, allowing > >> the VMM to use the GPA to retrieve additional details and print extra > >> information to aid in analysis? BTW, we will do this on the first fault > >> in cacheable, non-tagged memory even if there is no tagaccess in that > >> region. This can be further improved using the NoTagAccess series I > >> posted earlier, which ensures the memory fault exit occurs only on > >> actual tag access > >> > >> Something like below? > > > > Something like that, only with: > > > > - a capability informing userspace of this behaviour > > > > - a per-VM (or per-VMA) flag as a buy-in for that behaviour > > If we’re looking for a capability based control, could we tie that up to > FEAT_MTE_PERM? That’s what I did here: > > https://lore.kernel.org/all/20250110110023.2963795-1-aneesh.kumar@kernel.org > > That patch set also addresses the issue mentioned here. Let me know if > you think this is a better approach >From the patch linked above: | @@ -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; | } we also have the same ABI change every time FEAT_MTE_PERM is present. TBH, I'd rather have it from the start as per the patch in this thread, irrespective of FEAT_MTE_PERM. I'm fine, however, with better exit to VMM information though. If we don't want to confuse the VMMs, we could skip the kvm_vma_mte_allowed() check only for VM_ALLOW_ANY_UNCACHED and VM_PFNMAP vmas, maybe the latter only with FEAT_MTE_PERM. I don't think the VMM would get it wrong here since a VFIO mmap() would not support MTE anyway. -- Catalin