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Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1ljhH0-00GHMC-4p for linux-arm-kernel@lists.infradead.org; Thu, 20 May 2021 11:55:31 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id AC8AC101E; Thu, 20 May 2021 04:55:29 -0700 (PDT) Received: from [192.168.1.179] (unknown [172.31.20.19]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id EC54F3F73D; Thu, 20 May 2021 04:55:26 -0700 (PDT) From: Steven Price Subject: Re: [PATCH v12 3/8] arm64: mte: Sync tags for pages where PTE is untagged To: Catalin Marinas 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 References: <20210517123239.8025-1-steven.price@arm.com> <20210517123239.8025-4-steven.price@arm.com> <20210519180610.GE21619@arm.com> Message-ID: <3bac3a47-9f96-c7bf-e401-fdef60dcc9d8@arm.com> Date: Thu, 20 May 2021 12:55:21 +0100 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210519180610.GE21619@arm.com> Content-Language: en-GB X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210520_045530_314185_D312F2FB X-CRM114-Status: GOOD ( 32.51 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On 19/05/2021 19:06, Catalin Marinas wrote: > On Mon, May 17, 2021 at 01:32:34PM +0100, Steven Price wrote: >> A KVM guest could store tags in a page even if the VMM hasn't mapped >> the page with PROT_MTE. So when restoring pages from swap we will >> need to check to see if there are any saved tags even if !pte_tagged(). >> >> However don't check pages for which pte_access_permitted() returns false >> as these will not have been swapped out. >> >> Signed-off-by: Steven Price >> --- >> arch/arm64/include/asm/pgtable.h | 9 +++++++-- >> arch/arm64/kernel/mte.c | 16 ++++++++++++++-- >> 2 files changed, 21 insertions(+), 4 deletions(-) >> >> diff --git a/arch/arm64/include/asm/pgtable.h b/arch/arm64/include/asm/pgtable.h >> index 0b10204e72fc..275178a810c1 100644 >> --- a/arch/arm64/include/asm/pgtable.h >> +++ b/arch/arm64/include/asm/pgtable.h >> @@ -314,8 +314,13 @@ static inline void set_pte_at(struct mm_struct *mm, unsigned long addr, >> if (pte_present(pte) && pte_user_exec(pte) && !pte_special(pte)) >> __sync_icache_dcache(pte); >> >> - if (system_supports_mte() && >> - pte_present(pte) && pte_tagged(pte) && !pte_special(pte)) >> + /* >> + * If the PTE would provide user space access to the tags associated >> + * with it then ensure that the MTE tags are synchronised. Exec-only >> + * mappings don't expose tags (instruction fetches don't check tags). >> + */ >> + if (system_supports_mte() && pte_present(pte) && >> + pte_access_permitted(pte, false) && !pte_special(pte)) >> mte_sync_tags(ptep, pte); > > Looking at the mte_sync_page_tags() logic, we bail out early if it's the > old pte is not a swap one and the new pte is not tagged. So we only need > to call mte_sync_tags() if it's a tagged new pte or the old one is swap. > What about changing the set_pte_at() test to: > > if (system_supports_mte() && pte_present(pte) && !pte_special(pte) && > (pte_tagged(pte) || is_swap_pte(READ_ONCE(*ptep)))) > mte_sync_tags(ptep, pte); > > We can even change mte_sync_tags() to take the old pte directly: > > if (system_supports_mte() && pte_present(pte) && !pte_special(pte)) { > pte_t old_pte = READ_ONCE(*ptep); > if (pte_tagged(pte) || is_swap_pte(old_pte)) > mte_sync_tags(old_pte, pte); > } > > It would save a function call in most cases where the page is not > tagged. > Yes that looks like a good optimisation - although you've missed the pte_access_permitted() part of the check ;) The problem I hit is one of include dependencies: is_swap_pte() is defined (as a static inline) in include/linux/swapops.h. However the definition depends on pte_none()/pte_present() which are defined in pgtable.h - so there's a circular dependency. Open coding is_swap_pte() in set_pte_at() works, but it's a bit ugly. Any ideas on how to improve on the below? if (system_supports_mte() && pte_present(pte) && pte_access_permitted(pte, false) && !pte_special(pte)) { pte_t old_pte = READ_ONCE(*ptep); /* * We only need to synchronise if the new PTE has tags enabled * or if swapping in (in which case another mapping may have * set tags in the past even if this PTE isn't tagged). * (!pte_none() && !pte_present()) is an open coded version of * is_swap_pte() */ if (pte_tagged(pte) || (!pte_none(pte) && !pte_present(pte))) mte_sync_tags(old_pte, pte); } Steve _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel