Re: [PATCH] fs/resctrl: Fix MBM events being unconditionally enabled in mbm_event mode

[Babu Moger]

Hi Reinette,

On 10/6/25 20:23, Reinette Chatre wrote:
> 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?

It is independent from hardware side. I meant we still use legacy events 
from "default" mode.

> > > 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.


How about adding another check in get_rdt_mon_resources()?

if (rdt_cpu_has(X86_FEATURE_CQM_MBM_TOTAL)
    || rdt_is_feature_enabled(mbmtotal)) {
                resctrl_enable_mon_event(QOS_L3_MBM_TOTAL_EVENT_ID);
                ret = true;
        }

I need to take Tony's patch for this.

> > > 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.

Yes. It will be a problem in the that case.

I am not clear on using config option you mentioned above.

What about using the check resctrl_is_mon_event_enabled() in

resctrl_mbm_assign_mode_show() and resctrl_mbm_assign_mode_write() ?

Thanks
Babu