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a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:in-reply-to:message-id:mime-version :references:subject:from:to:cc:content-transfer-encoding; bh=wuCMvOpcLz1EO/bGDBjP5AoaMfs4wSC6ZWl6eaxBapc=; b=BF0cXJrL0wp4WoqLe8+bVATzRDwaRd54iNfUsuG4ODkw7ueVZxwE2PsPFJ2k5+rH9y 6HIFIZ+9faQmMVLdoVc84TaaweLRaoq/ciQ+Gp+GYpyj6tRQ+VosTacDHtjbWv6OZRUq tdA0Or9EVGNbBICL4dGONZ5EjF2OX22aCFatx0mU3/GFjZC5mwOJNIZ/9yGW6SZBrxSl KbGzCGVXzDXlqZpO6d9xz5hmO/+xNAqK/D0wkr5z3hHhiL6EJvGi/nRxdYjHFteitGkV juHQC0LSJIbKX3Zx8IcGSGeiPD4iOk8CX6VI2LCI4N2YME0TPrAyBcgUqua2jHGHvECk s3ew== X-Gm-Message-State: AOAM533PxDK+a/AzcWWyWaiY0P+LV0J6SG9Dqpobf8U1NNbEo3wSpWzL M1PcglaYYWfyRnliLgGMzyNBiLs= X-Google-Smtp-Source: ABdhPJxUJo0LJ4jCZzVUhiBC2Zc4BXc+gSLqpv2qKCrF71bDf6QI8JlN+M2MhmM+OYym8GFJj4PMUAQ= X-Received: from pcc-desktop.svl.corp.google.com ([2620:15c:2ce:200:8b00:104d:c249:d343]) (user=pcc job=sendgmr) by 2002:a25:2d57:: with SMTP id s23mr5185090ybe.302.1625094927802; Wed, 30 Jun 2021 16:15:27 -0700 (PDT) Date: Wed, 30 Jun 2021 16:15:09 -0700 In-Reply-To: <20210630231509.3773172-1-pcc@google.com> Message-Id: <20210630231509.3773172-5-pcc@google.com> Mime-Version: 1.0 References: <20210630231509.3773172-1-pcc@google.com> X-Mailer: git-send-email 2.32.0.93.g670b81a890-goog Subject: [PATCH v8 4/4] Documentation: document the preferred tag checking mode feature From: Peter Collingbourne To: Catalin Marinas , Vincenzo Frascino , Will Deacon Cc: Peter Collingbourne , Evgenii Stepanov , Szabolcs Nagy , Tejas Belagod , linux-arm-kernel@lists.infradead.org X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210630_161529_409001_3E6FB5E6 X-CRM114-Status: GOOD ( 17.59 ) 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: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Document the functionality added in the previous patches. Link: https://linux-review.googlesource.com/id/I48217cc3e8b8da33abc08cbaddc11cf4360a1b86 Signed-off-by: Peter Collingbourne Reviewed-by: Catalin Marinas --- .../arm64/memory-tagging-extension.rst | 48 ++++++++++++++++--- 1 file changed, 41 insertions(+), 7 deletions(-) diff --git a/Documentation/arm64/memory-tagging-extension.rst b/Documentation/arm64/memory-tagging-extension.rst index b540178a93f8..7b99c8f428eb 100644 --- a/Documentation/arm64/memory-tagging-extension.rst +++ b/Documentation/arm64/memory-tagging-extension.rst @@ -77,14 +77,20 @@ configurable behaviours: address is unknown). The user can select the above modes, per thread, using the -``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where -``flags`` contain one of the following values in the ``PR_MTE_TCF_MASK`` +``prctl(PR_SET_TAGGED_ADDR_CTRL, flags, 0, 0, 0)`` system call where ``flags`` +contains any number of the following values in the ``PR_MTE_TCF_MASK`` bit-field: -- ``PR_MTE_TCF_NONE`` - *Ignore* tag check faults +- ``PR_MTE_TCF_NONE``  - *Ignore* tag check faults + (ignored if combined with other options) - ``PR_MTE_TCF_SYNC`` - *Synchronous* tag check fault mode - ``PR_MTE_TCF_ASYNC`` - *Asynchronous* tag check fault mode +If no modes are specified, tag check faults are ignored. If a single +mode is specified, the program will run in that mode. If multiple +modes are specified, the mode is selected as described in the "Per-CPU +preferred tag checking modes" section below. + The current tag check fault mode can be read using the ``prctl(PR_GET_TAGGED_ADDR_CTRL, 0, 0, 0, 0)`` system call. @@ -120,13 +126,39 @@ in the ``PR_MTE_TAG_MASK`` bit-field. interface provides an include mask. An include mask of ``0`` (exclusion mask ``0xffff``) results in the CPU always generating tag ``0``. +Per-CPU preferred tag checking mode +----------------------------------- + +On some CPUs the performance of MTE in stricter tag checking modes +is similar to that of less strict tag checking modes. This makes it +worthwhile to enable stricter checks on those CPUs when a less strict +checking mode is requested, in order to gain the error detection +benefits of the stricter checks without the performance downsides. To +support this scenario, a privileged user may configure a stricter +tag checking mode as the CPU's preferred tag checking mode. + +The preferred tag checking mode for each CPU is controlled by +``/sys/devices/system/cpu/cpu/mte_tcf_preferred``, to which a +privileged user may write the value ``async`` or ``sync``. The default +preferred mode for each CPU is ``async``. + +To allow a program to potentially run in the CPU's preferred tag +checking mode, the user program may set multiple tag check fault mode +bits in the ``flags`` argument to the ``prctl(PR_SET_TAGGED_ADDR_CTRL, +flags, 0, 0, 0)`` system call. If the CPU's preferred tag checking +mode is in the task's set of provided tag checking modes (this will +always be the case at present because the kernel only supports two +tag checking modes, but future kernels may support more modes), that +mode will be selected. Otherwise, one of the modes in the task's mode +set will be selected in a currently unspecified manner. + Initial process state --------------------- On ``execve()``, the new process has the following configuration: - ``PR_TAGGED_ADDR_ENABLE`` set to 0 (disabled) -- Tag checking mode set to ``PR_MTE_TCF_NONE`` +- No tag checking modes are selected (tag check faults ignored) - ``PR_MTE_TAG_MASK`` set to 0 (all tags excluded) - ``PSTATE.TCO`` set to 0 - ``PROT_MTE`` not set on any of the initial memory maps @@ -251,11 +283,13 @@ Example of correct usage return EXIT_FAILURE; /* - * Enable the tagged address ABI, synchronous MTE tag check faults and - * allow all non-zero tags in the randomly generated set. + * Enable the tagged address ABI, synchronous or asynchronous MTE + * tag check faults (based on per-CPU preference) and allow all + * non-zero tags in the randomly generated set. */ if (prctl(PR_SET_TAGGED_ADDR_CTRL, - PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | (0xfffe << PR_MTE_TAG_SHIFT), + PR_TAGGED_ADDR_ENABLE | PR_MTE_TCF_SYNC | PR_MTE_TCF_ASYNC | + (0xfffe << PR_MTE_TAG_SHIFT), 0, 0, 0)) { perror("prctl() failed"); return EXIT_FAILURE;