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Date: Thu, 24 Nov 2022 14:21:25 +0000 From: James Morse <james.morse@....com> To: Valentin Schneider <vschneid@...hat.com>, x86@...nel.org, linux-kernel@...r.kernel.org Cc: Fenghua Yu <fenghua.yu@...el.com>, Reinette Chatre <reinette.chatre@...el.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>, H Peter Anvin <hpa@...or.com>, Babu Moger <Babu.Moger@....com>, shameerali.kolothum.thodi@...wei.com, D Scott Phillips OS <scott@...amperecomputing.com>, carl@...amperecomputing.com, lcherian@...vell.com, bobo.shaobowang@...wei.com, tan.shaopeng@...itsu.com, Jamie Iles <quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>, xingxin.hx@...nanolis.org, baolin.wang@...ux.alibaba.com, peternewman@...gle.com Subject: Re: [PATCH 07/18] x86/resctrl: Move CLOSID/RMID matching and setting to use helpers Hi Valentin, On 18/11/2022 15:49, Valentin Schneider wrote: > On 21/10/22 13:11, James Morse wrote: >> When switching tasks, the CLOSID and RMID that the new task should >> use are stored in struct task_struct. For x86 the CLOSID known by resctrl, >> the value in task_struct, and the value written to the CPU register are >> all the same thing. >> >> MPAM's CPU interface has two different PARTID's one for data accesses >> the other for instruction fetch. Storing resctrl's CLOSID value in >> struct task_struct implies the arch code knows whether resctrl is using >> CDP. >> >> Move the matching and setting of the struct task_struct properties >> to use helpers. This allows arm64 to store the hardware format of >> the register, instead of having to convert it each time. >> >> __rdtgroup_move_task()s use of READ_ONCE()/WRITE_ONCE() ensures torn >> values aren't seen as another CPU may schedule the task being moved >> while the value is being changed. MPAM has an additional corner-case >> here as the PMG bits extend the PARTID space. If the scheduler sees a >> new-CLOSID but old-RMID, the task will dirty an RMID that the limbo code >> is not watching causing an inaccurate count. x86's RMID are independent >> values, so the limbo code will still be watching the old-RMID in this >> circumstance. >> To avoid this, arm64 needs both the CLOSID/RMID WRITE_ONCE()d together. >> Both values must be provided together. > I think I remember something about this... Is that about having them > union'd and read/written as one? (just for my own curiosity) Yup. Many moons ago you pointed out that storing partid/pmg separately lets the remote CPUs load torn values. arm64 not only needs the values as a pair, it needs to read/write them as a pair. >> Because MPAM's RMID values are not unique, the CLOSID must be provided >> when matching the RMID. >> >> CC: Valentin Schneider <vschneid@...hat.com> (CC here also stands for celebrate-contribution!) >> Signed-off-by: James Morse <james.morse@....com> > > Reviewed-by: Valentin Schneider <vschneid@...hat.com> Thanks! James
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