Re: [PATCH v1 18/31] x86/resctrl: Allow resctrl_arch_mon_event_config_write() to return an error

[Reinette Chatre <reinette.chatre@intel.com>]

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