From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.3 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,USER_AGENT_SANE_1 autolearn=unavailable autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 8352FC433E1 for ; Tue, 30 Mar 2021 10:31:07 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 4EF10619A9 for ; Tue, 30 Mar 2021 10:31:07 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231549AbhC3Kag (ORCPT ); Tue, 30 Mar 2021 06:30:36 -0400 Received: from mail.kernel.org ([198.145.29.99]:43562 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231430AbhC3KaV (ORCPT ); Tue, 30 Mar 2021 06:30:21 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id F15E76195C; Tue, 30 Mar 2021 10:30:17 +0000 (UTC) Date: Tue, 30 Mar 2021 11:30:15 +0100 From: Catalin Marinas To: Steven Price Cc: Marc Zyngier , Will Deacon , James Morse , Julien Thierry , Suzuki K Poulose , kvmarm@lists.cs.columbia.edu, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Dave Martin , Mark Rutland , Thomas Gleixner , qemu-devel@nongnu.org, Juan Quintela , "Dr. David Alan Gilbert" , Richard Henderson , Peter Maydell , Haibo Xu , Andrew Jones Subject: Re: [PATCH v10 2/6] arm64: kvm: Introduce MTE VM feature Message-ID: <20210330103013.GD18075@arm.com> References: <20210312151902.17853-1-steven.price@arm.com> <20210312151902.17853-3-steven.price@arm.com> <20210327152324.GA28167@arm.com> <20210328122131.GB17535@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Mar 29, 2021 at 05:06:51PM +0100, Steven Price wrote: > On 28/03/2021 13:21, Catalin Marinas wrote: > > On Sat, Mar 27, 2021 at 03:23:24PM +0000, Catalin Marinas wrote: > > > On Fri, Mar 12, 2021 at 03:18:58PM +0000, Steven Price wrote: > > > > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c > > > > index 77cb2d28f2a4..b31b7a821f90 100644 > > > > --- a/arch/arm64/kvm/mmu.c > > > > +++ b/arch/arm64/kvm/mmu.c > > > > @@ -879,6 +879,22 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa, > > > > if (vma_pagesize == PAGE_SIZE && !force_pte) > > > > vma_pagesize = transparent_hugepage_adjust(memslot, hva, > > > > &pfn, &fault_ipa); > > > > + > > > > + if (fault_status != FSC_PERM && kvm_has_mte(kvm) && pfn_valid(pfn)) { > > > > + /* > > > > + * VM will be able to see the page's tags, so we must ensure > > > > + * they have been initialised. if PG_mte_tagged is set, tags > > > > + * have already been initialised. > > > > + */ > > > > + struct page *page = pfn_to_page(pfn); > > > > + unsigned long i, nr_pages = vma_pagesize >> PAGE_SHIFT; > > > > + > > > > + for (i = 0; i < nr_pages; i++, page++) { > > > > + if (!test_and_set_bit(PG_mte_tagged, &page->flags)) > > > > + mte_clear_page_tags(page_address(page)); > > > > + } > > > > + } > > > > > > This pfn_valid() check may be problematic. Following commit eeb0753ba27b > > > ("arm64/mm: Fix pfn_valid() for ZONE_DEVICE based memory"), it returns > > > true for ZONE_DEVICE memory but such memory is allowed not to support > > > MTE. > > > > Some more thinking, this should be safe as any ZONE_DEVICE would be > > mapped as untagged memory in the kernel linear map. It could be slightly > > inefficient if it unnecessarily tries to clear tags in ZONE_DEVICE, > > untagged memory. Another overhead is pfn_valid() which will likely end > > up calling memblock_is_map_memory(). > > > > However, the bigger issue is that Stage 2 cannot disable tagging for > > Stage 1 unless the memory is Non-cacheable or Device at S2. Is there a > > way to detect what gets mapped in the guest as Normal Cacheable memory > > and make sure it's only early memory or hotplug but no ZONE_DEVICE (or > > something else like on-chip memory)? If we can't guarantee that all > > Cacheable memory given to a guest supports tags, we should disable the > > feature altogether. > > In stage 2 I believe we only have two types of mapping - 'normal' or > DEVICE_nGnRE (see stage2_map_set_prot_attr()). Filtering out the latter is a > case of checking the 'device' variable, and makes sense to avoid the > overhead you describe. > > This should also guarantee that all stage-2 cacheable memory supports tags, > as kvm_is_device_pfn() is simply !pfn_valid(), and pfn_valid() should only > be true for memory that Linux considers "normal". That's the problem. With Anshuman's commit I mentioned above, pfn_valid() returns true for ZONE_DEVICE mappings (e.g. persistent memory, not talking about some I/O mapping that requires Device_nGnRE). So kvm_is_device_pfn() is false for such memory and it may be mapped as Normal but it is not guaranteed to support tagging. For user MTE, we get away with this as the MAP_ANONYMOUS requirement would filter it out while arch_add_memory() will ensure it's mapped as untagged in the linear map. See another recent fix for hotplugged memory: d15dfd31384b ("arm64: mte: Map hotplugged memory as Normal Tagged"). We needed to ensure that ZONE_DEVICE doesn't end up as tagged, only hoplugged memory. Both handled via arch_add_memory() in the arch code with ZONE_DEVICE starting at devm_memremap_pages(). > > > I now wonder if we can get a MAP_ANONYMOUS mapping of ZONE_DEVICE pfn > > > even without virtualisation. > > > > I haven't checked all the code paths but I don't think we can get a > > MAP_ANONYMOUS mapping of ZONE_DEVICE memory as we normally need a file > > descriptor. > > I certainly hope this is the case - it's the weird corner cases of device > drivers that worry me. E.g. I know i915 has a "hidden" mmap behind an ioctl > (see i915_gem_mmap_ioctl(), although this case is fine - it's MAP_SHARED). > Mali's kbase did something similar in the past. I think this should be fine since it's not a MAP_ANONYMOUS (we do allow MAP_SHARED to be tagged). -- Catalin