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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=N62IsDn/uNo2BIrbX/CKAnFDZcRj13ue8EMwn2XqY8U=; b=sZRS4z33HNhRzRXOZv6fToqq1CbkOOUFATpyM7noSzrE32RX3Ev9UtyWJmqbWNsG74 /oUB3TEkLBOVwyWCaEq8NOn1RZopMaCeRvht3zvc6no0E+FtyI3Q1hncZoHyON5QslMS Yo/MPbSmPUf5Fkd7BH78A9V9MTcBQZ+iEFLm1QXEQvdRBFtYfuNyniJkK64sTJEgn/kU uwJsBCo2CRuYhs7fSHFNtBVYCLXAeCPlL7HQCIsuGNZ9REELoyd3ydjth8MHPXjuKKXf +lct+ck6egQaPUf4Ki9mC2mMOanEWB8uQ7tGYRlGc6zoFKK5qbaoRZ7ld1ewU2OyYLNa XxNA== X-Gm-Message-State: AOAM530OI0umWl8O+YUEr/7i2+fMdCqky6Pq7OBUliVAjEW5wB+zwy85 hryPIg6v7thrsLG74wsx2SHSbng= X-Google-Smtp-Source: ABdhPJyIN6eBCRP+grvB50q+dC1uzVREqq5fg4GuVOrnfQ8g04jKDJBVZsToX9F4fI17Ew9KK2IXgvE= X-Received: from pcc-desktop.svl.corp.google.com ([2620:15c:2ce:200:99b9:48f7:dbe6:8670]) (user=pcc job=sendgmr) by 2002:a25:ada1:: with SMTP id z33mr9452281ybi.438.1626220097369; Tue, 13 Jul 2021 16:48:17 -0700 (PDT) Date: Tue, 13 Jul 2021 16:48:01 -0700 In-Reply-To: <20210713234801.3858018-1-pcc@google.com> Message-Id: <20210713234801.3858018-6-pcc@google.com> Mime-Version: 1.0 References: <20210713234801.3858018-1-pcc@google.com> X-Mailer: git-send-email 2.32.0.93.g670b81a890-goog Subject: [PATCH v10 5/5] 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, Greg Kroah-Hartman X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210713_164819_246759_83D836FA X-CRM114-Status: GOOD ( 20.18 ) 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 Acked-by: Will Deacon Acked-by: Will Deacon --- v10: - document that setting the sysfs node may not take effect immediately - unabbreviate month name v9: - add documentation for sysfs node under Documentation/ABI .../ABI/testing/sysfs-devices-system-cpu | 18 +++++++ .../arm64/memory-tagging-extension.rst | 48 ++++++++++++++++--- 2 files changed, 59 insertions(+), 7 deletions(-) diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu index fe13baa53c59..6e667338df9f 100644 --- a/Documentation/ABI/testing/sysfs-devices-system-cpu +++ b/Documentation/ABI/testing/sysfs-devices-system-cpu @@ -640,3 +640,21 @@ Description: SPURR ticks for cpuX when it was idle. This sysfs interface exposes the number of SPURR ticks for cpuX when it was idle. + +What: /sys/devices/system/cpu/cpuX/mte_tcf_preferred +Date: July 2021 +Contact: Linux ARM Kernel Mailing list +Description: Preferred MTE tag checking mode + + When a user program specifies more than one MTE tag checking + mode, this sysfs node is used to specify which mode should + be preferred when running on that CPU. Possible values: + + ================ ============================================== + "sync" Prefer synchronous mode + "async" Prefer asynchronous mode + ================ ============================================== + + Changes to this sysfs node may not take effect immediately. + + See also: Documentation/arm64/memory-tagging-extension.rst 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;