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

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Message-ID: <053a6879-f069-4073-a96f-dfacd7673d1f@intel.com>
Date: Tue, 17 Jun 2025 20:45:58 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....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 Babu,

Please modify subject prefix to "x86,fs/resctrl" since this adds new arch
API called by resctrl fs.

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"?

> 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."

>   */
>  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