linux-kernel - Re: [PATCH v17 25/33] fs/resctrl: Provide interface to update the event configurations

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Message-ID: <c6c1df22-e3df-4acd-a93a-f2972cac9793@intel.com>
Date: Wed, 3 Sep 2025 10:55:04 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: <babu.moger@....com>, <corbet@....net>, <tony.luck@...el.com>,
	<tglx@...utronix.de>, <mingo@...hat.com>, <bp@...en8.de>,
	<dave.hansen@...ux.intel.com>
CC: <Dave.Martin@....com>, <james.morse@....com>, <x86@...nel.org>,
	<hpa@...or.com>, <akpm@...ux-foundation.org>, <rostedt@...dmis.org>,
	<paulmck@...nel.org>, <pawan.kumar.gupta@...ux.intel.com>, <kees@...nel.org>,
	<arnd@...db.de>, <fvdl@...gle.com>, <seanjc@...gle.com>,
	<thomas.lendacky@....com>, <yosry.ahmed@...ux.dev>, <xin@...or.com>,
	<sohil.mehta@...el.com>, <kai.huang@...el.com>, <xiaoyao.li@...el.com>,
	<peterz@...radead.org>, <mario.limonciello@....com>, <xin3.li@...el.com>,
	<perry.yuan@....com>, <chang.seok.bae@...el.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<peternewman@...gle.com>, <eranian@...gle.com>, <gautham.shenoy@....com>
Subject: Re: [PATCH v17 25/33] fs/resctrl: Provide interface to update the
 event configurations

Hi Babu,

On 9/3/25 10:38 AM, Moger, Babu wrote:
> On 9/2/25 21:41, Reinette Chatre wrote:
>> On 8/14/25 7:25 PM, Babu Moger wrote:

>>> +
>>> +ssize_t event_filter_write(struct kernfs_open_file *of, char *buf, size_t nbytes,
>>> +			   loff_t off)
>>> +{
>>> +	struct mon_evt *mevt = rdt_kn_parent_priv(of->kn);
>>> +	struct rdt_resource *r;
>>> +	u32 evt_cfg = 0;
>>> +	int ret = 0;
>>> +
>>> +	/* Valid input requires a trailing newline */
>>> +	if (nbytes == 0 || buf[nbytes - 1] != '\n')
>>> +		return -EINVAL;
>>> +
>>> +	buf[nbytes - 1] = '\0';
>>> +
>>> +	cpus_read_lock();
>>> +	mutex_lock(&rdtgroup_mutex);
>>> +
>>> +	rdt_last_cmd_clear();
>>> +
>>> +	r = resctrl_arch_get_resource(mevt->rid);
>>> +	if (!resctrl_arch_mbm_cntr_assign_enabled(r)) {
>>> +		rdt_last_cmd_puts("mbm_event counter assignment mode is not enabled\n");
>>> +		ret = -EINVAL;
>>> +		goto out_unlock;
>>> +	}
>>> +
>>> +	ret = resctrl_parse_mem_transactions(buf, &evt_cfg);
>>> +	if (!ret && mevt->evt_cfg != evt_cfg) {
>>
>> ... is evt_cfg of 0 (a) a valid value (that will not cause hardware to fault) and
>> (b) a reasonable value to allow? 
>>
> 
> The value evt_cfg = 0 is valid and permitted for both ABMC and BMEC. I
> have confirmed here through verification and testing. In that case, the
> event counter will not be monitoring anything.

Thank you for checking.

I do not know what a use case for this would be but I also do not see why kernel
should prevent user space from doing this. Looks like event_filter_show() will then
just print a '\n' that is expected. Are counters expected to always return 0 in this
case or will they return an error? I am not clear on what qualifies as "invalid counter
configuration" for ABMC that results in RMID_VAL_ERROR.

Reinette