linux-kernel - Re: [PATCH v6 3/8] x86/resctrl: Detect io

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Message-ID: <58ea6834-7765-434f-8d25-ea71e1f2d5e4@amd.com>
Date: Wed, 18 Jun 2025 14:27:13 -0500
From: "Moger, Babu" <babu.moger@....com>
To: Reinette Chatre <reinette.chatre@...el.com>, corbet@....net,
 tony.luck@...el.com, Dave.Martin@....com, james.morse@....com,
 tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com
Cc: x86@...nel.org, hpa@...or.com, akpm@...ux-foundation.org,
 paulmck@...nel.org, rostedt@...dmis.org, thuth@...hat.com, ardb@...nel.org,
 gregkh@...uxfoundation.org, seanjc@...gle.com, thomas.lendacky@....com,
 pawan.kumar.gupta@...ux.intel.com, perry.yuan@....com,
 yosry.ahmed@...ux.dev, kai.huang@...el.com, xiaoyao.li@...el.com,
 peterz@...radead.org, kan.liang@...ux.intel.com, mario.limonciello@....com,
 xin3.li@...el.com, sohil.mehta@...el.com, chang.seok.bae@...el.com,
 andrew.cooper3@...rix.com, ebiggers@...gle.com, ak@...ux.intel.com,
 xin@...or.com, linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 3/8] x86/resctrl: Detect io_alloc feature

Hi Reinette,

On 6/17/25 22:45, Reinette Chatre wrote:
> Hi Babu,
> 
> Please modify subject prefix to "x86,fs/resctrl" since this adds new arch
> API called by resctrl fs.

Sure.

> 
> On 6/11/25 2:23 PM, Babu Moger wrote:
>> Smart Data Cache Injection (SDCI) is a mechanism that enables direct
>> insertion of data from I/O devices into the L3 cache. It can the demands
> 
> "It can the demands" -> "It can reduce demands"?

Sure.

> 
>> on DRAM bandwidth and reduces latency to the processor consuming the I/O
>> data.
>>
>> Introduce cache resource property "io_alloc_capable" that an architecture
>> can set if a portion of the L3 cache can be allocated for I/O traffic.
> 
> I think "L3" should be dropped because the cache resource property is not
> unique to L3. It is a property of all cache resources. 
> 
>> Set this property on x86 systems that support SDCIAE (L3 Smart Data Cache
>> Injection Allocation Enforcement).
> 
> Here it can be mentioned that this property is only set for the L3 cache
> resource on systems that supports SDCIAE.
> 
>>
>> Signed-off-by: Babu Moger <babu.moger@....com>
>> ---
>> v6: No changes.
>>
>> v5: No changes.
>>
>> v4: Updated the commit message and code comment based on feedback.
>>
>> v3: Rewrote commit log. Changed the text to bit generic than the AMD specific.
>>     Renamed the rdt_get_sdciae_alloc_cfg() to rdt_set_io_alloc_capable().
>>     Removed leftover comment from v2.
>>
>> v2: Changed sdciae_capable to io_alloc_capable to make it generic feature.
>>     Also moved the io_alloc_capable in struct resctrl_cache.
>> ---
>>  arch/x86/kernel/cpu/resctrl/core.c | 7 +++++++
>>  include/linux/resctrl.h            | 3 +++
>>  2 files changed, 10 insertions(+)
>>
>> diff --git a/arch/x86/kernel/cpu/resctrl/core.c b/arch/x86/kernel/cpu/resctrl/core.c
>> index 326c679ade5c..a3d174362249 100644
>> --- a/arch/x86/kernel/cpu/resctrl/core.c
>> +++ b/arch/x86/kernel/cpu/resctrl/core.c
>> @@ -274,6 +274,11 @@ static void rdt_get_cdp_config(int level)
>>  	rdt_resources_all[level].r_resctrl.cdp_capable = true;
>>  }
>>  
>> +static void rdt_set_io_alloc_capable(struct rdt_resource *r)
>> +{
>> +	r->cache.io_alloc_capable = true;
>> +}
>> +
>>  static void rdt_get_cdp_l3_config(void)
>>  {
>>  	rdt_get_cdp_config(RDT_RESOURCE_L3);
>> @@ -840,6 +845,8 @@ static __init bool get_rdt_alloc_resources(void)
>>  		rdt_get_cache_alloc_cfg(1, r);
>>  		if (rdt_cpu_has(X86_FEATURE_CDP_L3))
>>  			rdt_get_cdp_l3_config();
>> +		if (rdt_cpu_has(X86_FEATURE_SDCIAE))
>> +			rdt_set_io_alloc_capable(r);
>>  		ret = true;
>>  	}
>>  	if (rdt_cpu_has(X86_FEATURE_CAT_L2)) {
>> diff --git a/include/linux/resctrl.h b/include/linux/resctrl.h
>> index 9ba771f2ddea..0e8641e41100 100644
>> --- a/include/linux/resctrl.h
>> +++ b/include/linux/resctrl.h
>> @@ -191,6 +191,8 @@ struct rdt_mon_domain {
>>   * @arch_has_sparse_bitmasks:	True if a bitmask like f00f is valid.
>>   * @arch_has_per_cpu_cfg:	True if QOS_CFG register for this cache
>>   *				level has CPU scope.
>> + * @io_alloc_capable:	True if portion of the cache can be allocated
>> + *			for I/O traffic.
> 
> Reading this again I think the description can be improved since technically
> resctrl does not allocate a portion of cache but instead configures the
> portion of cache that device can allocate into.
> So perhaps this can be: "True if portion of the cache can be configured
> for I/O traffic allocation."

Sure.

> 
>>   */
>>  struct resctrl_cache {
>>  	unsigned int	cbm_len;
>> @@ -198,6 +200,7 @@ struct resctrl_cache {
>>  	unsigned int	shareable_bits;
>>  	bool		arch_has_sparse_bitmasks;
>>  	bool		arch_has_per_cpu_cfg;
>> +	bool		io_alloc_capable;
>>  };
>>  
>>  /**
> 
> Reinette
> 

-- 
Thanks
Babu Moger