linux-kernel - Re: [PATCH v1 18/31] x86/resctrl: Allow resctrl_arch_mon_event_config

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Date: Wed, 17 Apr 2024 22:19:31 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Dave Martin <Dave.Martin@....com>
CC: James Morse <james.morse@....com>, <x86@...nel.org>,
	<linux-kernel@...r.kernel.org>, Fenghua Yu <fenghua.yu@...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>, <baolin.wang@...ux.alibaba.com>, Jamie Iles
	<quic_jiles@...cinc.com>, Xin Hao <xhao@...ux.alibaba.com>,
	<peternewman@...gle.com>, <dfustini@...libre.com>, <amitsinght@...vell.com>,
	David Hildenbrand <david@...hat.com>, Rex Nie <rex.nie@...uarmicro.com>
Subject: Re: [PATCH v1 18/31] x86/resctrl: Allow
 resctrl_arch_mon_event_config_write() to return an error

Hi Dave,

On 4/17/2024 7:42 AM, Dave Martin wrote:
> Hi Rainette,
> 
> On Thu, Apr 11, 2024 at 10:39:37AM -0700, Reinette Chatre wrote:
>> Hi Dave,
>>
>> On 4/11/2024 7:17 AM, Dave Martin wrote:
>>> On Mon, Apr 08, 2024 at 08:23:36PM -0700, Reinette Chatre wrote:
>>>> Hi James,
>>>>
>>>> On 3/21/2024 9:50 AM, James Morse wrote:
>>>>> resctrl_arch_mon_event_config_write() writes a bitmap of events provided
>>>>> by user-space into the configuration register for the monitors.
>>>>>
>>>>> This assumes that all architectures support all the features each bit
>>>>> corresponds to.
>>>>>
>>>>> MPAM can filter monitors based on read, write, or both, but there are
>>>>> many more options in the existing bitmap. To allow this interface to
>>>>> work for machines with MPAM, allow the architecture helper to return
>>>>> an error if an incompatible bitmap is set.
>>>>>
>>>>> When valid values are provided, there is no change in behaviour. If
>>>>> an invalid value is provided, currently it is silently ignored, but
>>>>> last_cmd_status is updated. After this change, the parser will stop
>>>>> at the first invalid value and return an error to user-space. This
>>>>> matches the way changes to the schemata file are made.
>>>>>
>>>>
>>>> Is this needed? With move of mbm_cfg_mask to rdt_resource I expect
>>>> MPAM would use it to set what the valid values are. With that done,
>>>> when user space provides a value, mon_config_write() compares user
>>>> provided value against mbm_cfg_mask and will already return early
>>>> (before attempting to write to hardware) with error
>>>> if value is not supported. This seems to accomplish the goal of this
>>>> patch?
>>>
>>> This sounds plausible.
>>>
>>> In a recent snapshot of James' MPAM code, it looks like we could be
>>> initialising rdt_resource::mbm_cfg_mask when setting up the rdt_resource
>>> struct for resctrl, though in fact this information is captured
>>> differently right now.  I'm sure why (though James may have a
>>> reason). [1]
>>>
>>> I don't see an obvious reason though why we couldn't set mbm_cfg_mask
>>> and detect bad config values globally in mon_config_write(), the same as
>>> for the existing AMD BMEC case.
>>>
>>> Nothing in the MPAM architecture stops hardware vendors from randomly
>>> implementing different capabilities in different components of the
>>> system, but provided that we only expose the globally supported subset
>>> of event filtering capabilities to resctrl this approach looks workable.
>>> This consistent with the James' MPAM code deals with other feature
>>> mismatches across the system today.
>>>
>>> [1] https://git.kernel.org/pub/scm/linux/kernel/git/morse/linux.git/tree/drivers/platform/mpam/mpam_resctrl.c?h=mpam/snapshot/v6.7-rc2#n730
>>
>> My response was based on what I understood from the goal of this change
>> as described by the changelog. The patch does not appear to match with
>> the goals stated in changelog.
>>
>> As I understand the patch it aims to detect when there is an invalid
>> event id. It is not possible for this scenario to occur because this code
>> is always called with a valid event id.
>>
>> Reinette
> 
> I guess this will need discussion with James.  FWIW, my impression was
> that the real goal of this patch was to allow a bad event config to be
> detected at cross-call time and reported asynchronously.  Changes
> elsewhere look to be there just to make error reporting consistent for
> other existing paths too.

How do you interpret "bad event config"?

As I understand it, this patch only sets an error in one scenario:

	 	index = mon_event_config_index_get(mon_info->evtid);
	 	if (index == INVALID_CONFIG_INDEX) {
 			pr_warn_once("Invalid event id %d\n", mon_info->evtid);
			mon_info->err = -EINVAL;
	 		return;
 		}

When will mon_info->evtid be anything but QOS_L3_MBM_TOTAL_EVENT_ID or
QOS_L3_MBM_LOCAL_EVENT_ID? 

Reinette