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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 94FAFECD983 for ; Thu, 5 Feb 2026 16:16:44 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=SIGtsCjedCkQ2XcBx++sVU0LQGhcodgsYVu5gusy8bY=; b=YHA3nL8xU5J/okgbKit/qlXnKh RyNR/24XuCJWObbFnS72tJ+4ibfF4heNyYgQjlVcJoSQxnAeB2JQWlrej8zG/QneEVX4/SptWjiv1 aXW0WxfUEb8a3DVZhKMW1PYzgKy1r6TFtUebijmjhhsfVY0qQIVmXTm18LoxZuTkYceCBM0dE6kgx tlnRuAelJuEAlQ9NIfln0ZhrmZFZoVSgpi7ZY4GXRn2imsADNEjjSl+LQWmDPnbQH4GLU8CsDeK+A y+q8LCrzTroqLhcLwkbE054upMckQ8TRqXu6g1O9NyHIfbctRZe9DTUcfqBkerF5EVhM/NVtTmdaK XfNzudEQ==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vo221-0000000ADXu-12fH; Thu, 05 Feb 2026 16:16:37 +0000 Received: from foss.arm.com ([217.140.110.172]) by bombadil.infradead.org with esmtp (Exim 4.98.2 #2 (Red Hat Linux)) id 1vo21x-0000000ADXP-2hgL for linux-arm-kernel@lists.infradead.org; Thu, 05 Feb 2026 16:16:35 +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 56515339; Thu, 5 Feb 2026 08:16:25 -0800 (PST) Received: from arm.com (arrakis.cambridge.arm.com [10.1.197.46]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id E314C3F632; Thu, 5 Feb 2026 08:16:25 -0800 (PST) Date: Thu, 5 Feb 2026 16:16:23 +0000 From: Catalin Marinas To: Ben Horgan Cc: amitsinght@marvell.com, baisheng.gao@unisoc.com, baolin.wang@linux.alibaba.com, carl@os.amperecomputing.com, dave.martin@arm.com, david@kernel.org, dfustini@baylibre.com, fenghuay@nvidia.com, gshan@redhat.com, james.morse@arm.com, jonathan.cameron@huawei.com, kobak@nvidia.com, lcherian@marvell.com, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, peternewman@google.com, punit.agrawal@oss.qualcomm.com, quic_jiles@quicinc.com, reinette.chatre@intel.com, rohit.mathew@arm.com, scott@os.amperecomputing.com, sdonthineni@nvidia.com, tan.shaopeng@fujitsu.com, xhao@linux.alibaba.com, will@kernel.org, corbet@lwn.net, maz@kernel.org, oupton@kernel.org, joey.gouly@arm.com, suzuki.poulose@arm.com, kvmarm@lists.linux.dev, zengheng4@huawei.com, linux-doc@vger.kernel.org, Shaopeng Tan Subject: Re: [PATCH v4 04/41] arm64: mpam: Context switch the MPAM registers Message-ID: References: <20260203214342.584712-1-ben.horgan@arm.com> <20260203214342.584712-5-ben.horgan@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260203214342.584712-5-ben.horgan@arm.com> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20260205_081633_803031_30621355 X-CRM114-Status: GOOD ( 27.98 ) 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 On Tue, Feb 03, 2026 at 09:43:05PM +0000, Ben Horgan wrote: > From: James Morse > > MPAM allows traffic in the SoC to be labeled by the OS, these labels are > used to apply policy in caches and bandwidth regulators, and to monitor > traffic in the SoC. The label is made up of a PARTID and PMG value. The x86 > equivalent calls these CLOSID and RMID, but they don't map precisely. > > MPAM has two CPU system registers that is used to hold the PARTID and PMG > values that traffic generated at each exception level will use. These can > be set per-task by the resctrl file system. (resctrl is the defacto > interface for controlling this stuff). > > Add a helper to switch this. > > struct task_struct's separate CLOSID and RMID fields are insufficient to > implement resctrl using MPAM, as resctrl can change the PARTID (CLOSID) and > PMG (sort of like the RMID) separately. On x86, the rmid is an independent > number, so a race that writes a mismatched closid and rmid into hardware is > benign. On arm64, the pmg bits extend the partid. > (i.e. partid-5 has a pmg-0 that is not the same as partid-6's pmg-0). In > this case, mismatching the values will 'dirty' a pmg value that resctrl > believes is clean, and is not tracking with its 'limbo' code. > > To avoid this, the partid and pmg are always read and written as a > pair. This requires a new u64 field. In struct task_struct there are two > u32, rmid and closid for the x86 case, but as we can't use them here do > something else. Add this new field, mpam_partid_pmg, to struct thread_info > to avoid adding more architecture specific code to struct task_struct. > Always use READ_ONCE()/WRITE_ONCE() when accessing this field. > > Resctrl allows a per-cpu 'default' value to be set, this overrides the > values when scheduling a task in the default control-group, which has > PARTID 0. The way 'code data prioritisation' gets emulated means the > register value for the default group needs to be a variable. > > The current system register value is kept in a per-cpu variable to avoid > writing to the system register if the value isn't going to change. Writes > to this register may reset the hardware state for regulating bandwidth. > > Finally, there is no reason to context switch these registers unless there > is a driver changing the values in struct task_struct. Hide the whole thing > behind a static key. This also allows the driver to disable MPAM in > response to errors reported by hardware. Move the existing static key to > belong to the arch code, as in the future the MPAM driver may become a > loadable module. > > All this should depend on whether there is an MPAM driver, hide it behind > CONFIG_ARM64_MPAM. > > Tested-by: Gavin Shan > Tested-by: Shaopeng Tan > Tested-by: Peter Newman > CC: Amit Singh Tomar > Reviewed-by: Jonathan Cameron > Signed-off-by: James Morse > Reviewed-by: Gavin Shan > Signed-off-by: Ben Horgan Reviewed-by: Catalin Marinas