From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from frasgout.his.huawei.com (frasgout.his.huawei.com [185.176.79.56]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 2196233121A for ; Thu, 18 Dec 2025 10:35:57 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=185.176.79.56 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766054161; cv=none; b=nb3wMHFt6LuPjQciEqAxyzuPA9Dw5balOV8bOdj+izahfpAIbklZOwIJdwR20IaHXWYiPrNW6k/LxB7rGmBLl3sSGo/cZt3YK89S73VAovETrCaCW/bbdnvgxyag3OdUW+i8dVY7LKkcmf5uXbmJwfiuQ/f7tQoaSvaNCJvC+vE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1766054161; c=relaxed/simple; bh=+H3Tw9cGBciZgkmIqbFNCLWgo+UbDZRKDOo5kvJ0loQ=; h=Date:From:To:CC:Subject:Message-ID:In-Reply-To:References: MIME-Version:Content-Type; b=u1MeodINFg152v+eo0wtu7bI3ISLmm4nvdT3rguh9q9S0BG02rPFqgHpQcKUqlCVijfoxeImrt0O0v/sdR+iLTJeOgD0SmfIINmXMPpd/V7cpcsYjeZrQaizxHUBM/BxiSiqKYLfaH9ExPL2fr1mhIy9PvOA3UKfsny1FOXBD4o= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com; spf=pass smtp.mailfrom=huawei.com; arc=none smtp.client-ip=185.176.79.56 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=huawei.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huawei.com Received: from mail.maildlp.com (unknown [172.18.224.107]) by frasgout.his.huawei.com (SkyGuard) with ESMTPS id 4dX6WB4YVHzJ46F1; Thu, 18 Dec 2025 18:35:18 +0800 (CST) Received: from dubpeml100005.china.huawei.com (unknown [7.214.146.113]) by mail.maildlp.com (Postfix) with ESMTPS id 73AC140570; Thu, 18 Dec 2025 18:35:49 +0800 (CST) Received: from localhost (10.203.177.15) by dubpeml100005.china.huawei.com (7.214.146.113) with Microsoft SMTP Server (version=TLS1_2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id 15.2.1544.36; Thu, 18 Dec 2025 10:35:48 +0000 Date: Thu, 18 Dec 2025 10:35:46 +0000 From: Jonathan Cameron To: James Morse CC: , , D Scott Phillips OS , , , , , , Jamie Iles , "Xin Hao" , , , , David Hildenbrand , Dave Martin , Koba Ko , Shanker Donthineni , , , Gavin Shan , Ben Horgan , , , Punit Agrawal Subject: Re: [RFC PATCH 01/38] arm64: mpam: Context switch the MPAM registers Message-ID: <20251218103546.000070fd@huawei.com> In-Reply-To: <20251205215901.17772-2-james.morse@arm.com> References: <20251205215901.17772-1-james.morse@arm.com> <20251205215901.17772-2-james.morse@arm.com> X-Mailer: Claws Mail 4.3.0 (GTK 3.24.42; x86_64-w64-mingw32) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit X-ClientProxiedBy: lhrpeml100011.china.huawei.com (7.191.174.247) To dubpeml100005.china.huawei.com (7.214.146.113) On Fri, 5 Dec 2025 21:58:24 +0000 James Morse wrote: > 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. > Instead of making struct task_struct's closid and rmid fields an > endian-unsafe union, add the value to struct thread_info and 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_MPAM. > > CC: Amit Singh Tomar > Signed-off-by: James Morse > diff --git a/arch/arm64/include/asm/mpam.h b/arch/arm64/include/asm/mpam.h > new file mode 100644 > index 000000000000..86a55176f884 > --- /dev/null > +++ b/arch/arm64/include/asm/mpam.h > @@ -0,0 +1,74 @@ ... > +/* > + * The resctrl filesystem writes to the partid/pmg values for threads and CPUs, > + * which may race with reads in __mpam_sched_in(). Ensure only one of the old > + * or new values are used. Particular care should be taken with the pmg field > + * as __mpam_sched_in() may read a partid and pmg that don't match, causing > + * this value to be stored with cache allocations, despite being considered > + * 'free' by resctrl. > + * > + * A value in struct thread_info is used instead of struct task_struct as the > + * cpu's u64 register format is used, but struct task_struct has two u32'. This comment probably wants to provide a little more info if it is to be useful, Is it a reference to the closid and rmid fields under CONFIG_X86_CPU_RESCTRL? I'm not immediately understanding why that matters given you could slap a union on it without (I think) resulting in anything else moving. Now having it in thread_info moves it into arch header territory so might make sense for that reason. > + */ > +static inline u64 mpam_get_regval(struct task_struct *tsk) > +{ > +#ifdef CONFIG_ARM64_MPAM > + return READ_ONCE(task_thread_info(tsk)->mpam_partid_pmg); > +#else > + return 0; > +#endif > +}