linux-kernel - Re: [PATCH v3 3/7] x86/resctrl: Detect io

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Message-ID: <fff9df3e-34ae-4aa0-8847-130a4ffd8fcf@intel.com>
Date: Fri, 21 Mar 2025 15:51:25 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: Babu Moger <babu.moger@....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>, <thuth@...hat.com>, <rostedt@...dmis.org>,
	<xiongwei.song@...driver.com>, <pawan.kumar.gupta@...ux.intel.com>,
	<jpoimboe@...nel.org>, <daniel.sneddon@...ux.intel.com>,
	<thomas.lendacky@....com>, <perry.yuan@....com>, <sandipan.das@....com>,
	<kai.huang@...el.com>, <seanjc@...gle.com>, <xin3.li@...el.com>,
	<ebiggers@...gle.com>, <andrew.cooper3@...rix.com>,
	<mario.limonciello@....com>, <tan.shaopeng@...itsu.com>,
	<james.morse@....com>, <tony.luck@...el.com>, <peternewman@...gle.com>,
	<linux-doc@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	<eranian@...gle.com>, <corbet@....net>
Subject: Re: [PATCH v3 3/7] x86/resctrl: Detect io_alloc feature

Hi Babu,

On 1/30/25 1:20 PM, Babu Moger wrote:
> "io_alloc" feature is a mechanism that enables direct insertion of data
> from I/O devices into the L3 cache. By directly caching data from I/O
> devices rather than first storing the I/O data in DRAM, feature reduces
> the demands on DRAM bandwidth and reduces latency to the processor
> consuming the I/O data.

This provides good context but the changelog does not mention what this patch
does.

An idea to get started (please improve):
	Data from I/O devices can be inserted directly into L3 cache. This
	reduces 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. Set this property on x86 systems that support SDCIAE.  

> 
> Signed-off-by: Babu Moger <babu.moger@....com>
> ---
> 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 c2450cd52511..1ebdb2dcc009 100644
> --- a/arch/x86/kernel/cpu/resctrl/core.c
> +++ b/arch/x86/kernel/cpu/resctrl/core.c
> @@ -306,6 +306,11 @@ 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);
> @@ -931,6 +936,8 @@ 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 d94abba1c716..dbe6461f3fbc 100644
> --- a/include/linux/resctrl.h
> +++ b/include/linux/resctrl.h
> @@ -129,6 +129,8 @@ 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 L3 cache can be allocated
> + *			for I/O traffic.

Enforcing that this should be for L3 cache is confusing. On a system with
L3 and L2 cache resources each resource will be described by the properties in
struct resctrl_cache for particular resource. We do not want to set "io_alloc_capable"
to true in the L2's struct if the L3 cache supports this feature.

This can just be: "True if portion of the cache can be allocated for I/O traffic"


>   */
>  struct resctrl_cache {
>  	unsigned int	cbm_len;
> @@ -136,6 +138,7 @@ struct resctrl_cache {
>  	unsigned int	shareable_bits;
>  	bool		arch_has_sparse_bitmasks;
>  	bool		arch_has_per_cpu_cfg;
> +	bool		io_alloc_capable;
>  };
>  
>  /**

Reinette