linux-kernel - Re: [PATCH] fs/resctrl: Fix MBM events being unconditionally enabled in mbm

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Message-ID: <2cdc5b52-a00c-4772-8221-8d98b787722a@intel.com>
Date: Mon, 6 Oct 2025 18:23:15 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <tony.luck@...el.com>, <Dave.Martin@....com>,
	<james.morse@....com>, <dave.hansen@...ux.intel.com>, <bp@...en8.de>
CC: <kas@...nel.org>, <rick.p.edgecombe@...el.com>,
	<linux-kernel@...r.kernel.org>, <x86@...nel.org>,
	<linux-coco@...ts.linux.dev>, <kvm@...r.kernel.org>
Subject: Re: [PATCH] fs/resctrl: Fix MBM events being unconditionally enabled
 in mbm_event mode

Hi Babu,

On 10/6/25 1:38 PM, Moger, Babu wrote:
> Hi Reinette,
> 
> On 10/6/25 12:56, Reinette Chatre wrote:
>> Hi Babu,
>>
>> On 9/30/25 1:26 PM, Babu Moger wrote:
>>> resctrl features can be enabled or disabled using boot-time kernel
>>> parameters. To turn off the memory bandwidth events (mbmtotal and
>>> mbmlocal), users need to pass the following parameter to the kernel:
>>> "rdt=!mbmtotal,!mbmlocal".
>>
>> ah, indeed ... although, the intention behind the mbmtotal and mbmlocal kernel
>> parameters was to connect them to the actual hardware features identified
>> by X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL respectively.
>>
>>
>>> Found that memory bandwidth events (mbmtotal and mbmlocal) cannot be
>>> disabled when mbm_event mode is enabled. resctrl_mon_resource_init()
>>> unconditionally enables these events without checking if the underlying
>>> hardware supports them.
>>
>> Technically this is correct since if hardware supports ABMC then the
>> hardware is no longer required to support X86_FEATURE_CQM_MBM_TOTAL and
>> X86_FEATURE_CQM_MBM_LOCAL in order to provide mbm_total_bytes
>> and mbm_local_bytes. 
>>
>> I can see how this may be confusing to user space though ...
>>
>>>
>>> Remove the unconditional enablement of MBM features in
>>> resctrl_mon_resource_init() to fix the problem. The hardware support
>>> verification is already done in get_rdt_mon_resources().
>>
>> I believe by "hardware support" you mean hardware support for 
>> X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL. Wouldn't a fix like
>> this then require any system that supports ABMC to also support
>> X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL to be able to 
>> support mbm_total_bytes and mbm_local_bytes?
> 
> Yes. That is correct. Right now, ABMC and X86_FEATURE_CQM_MBM_TOTAL/
> X86_FEATURE_CQM_MBM_LOCAL are kind of tightly coupled. We have not clearly
> separated the that.

Are you speaking from resctrl side since from what I understand these are
independent features from the hardware side?

>> This problem seems to be similar to the one solved by [1] since
>> by supporting ABMC there is no "hardware does not support mbmtotal/mbmlocal"
>> but instead there only needs to be a check if the feature has been disabled
>> by command line. That is, add a rdt_is_feature_enabled() check to the
>> existing "!resctrl_is_mon_event_enabled()" check?
> 
> Enable or disable needs to be done at get_rdt_mon_resources(). It needs to
> be done early in  the initialization before calling domain_add_cpu() where
> event data structures (mbm_states aarch_mbm_states) are allocated.

Good point. My mistake to suggest the event should be enabled by
resctrl fs.

> 
>>
>> But wait ... I think there may be a bigger problem when considering systems
>> that support ABMC but not X86_FEATURE_CQM_MBM_TOTAL and X86_FEATURE_CQM_MBM_LOCAL.
>> Shouldn't resctrl prevent such a system from switching to "default" 
>> mbm_assign_mode? Otherwise resctrl will happily let such a system switch
>> to default mode and when user attempts to read an event file resctrl will
>> attempt to read it via MSRs that are not supported.
>> Looks like ABMC may need something similar to CONFIG_RESCTRL_ASSIGN_FIXED
>> to handle this case in show() while preventing user space from switching to
>> "default" mode on write()?
> 
> This may not be an issue right now. When X86_FEATURE_CQM_MBM_TOTAL and
> X86_FEATURE_CQM_MBM_LOCAL are not supported then mon_data files of these
> events are not created.

By "right now" I assume you mean the current implementation? I think your statement
assumes that no CPUs come or go after resctrl_mon_resource_init() enables the MBM events?
Current implementation will enable MBM events if ABMC is supported. When the
first CPU of a domain comes online after that then resctrl will create the mon_data
files. These files will remain if a user then switches to default mode and if
the user then attempts to read one of these counters then I expect problems.

Reinette