linux-kernel - Re: [PATCH 1/3] x86/resctrl: Enable non-contiguous bits in Intel CAT

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Message-ID: <ob4e7sb4pfitjn7arlt7f6be3hvdoyp5yecb4o5ub3i36u2qhv@4s6vzu5nk342>
Date:   Tue, 12 Sep 2023 08:53:19 +0200
From:   Maciej Wieczór-Retman 
        <maciej.wieczor-retman@...el.com>
To:     Reinette Chatre <reinette.chatre@...el.com>
CC:     Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] x86/resctrl: Enable non-contiguous bits in Intel CAT

Hello,

On 2023-09-11 at 13:05:16 -0700, Reinette Chatre wrote:
>Hi Maciej,
>> diff --git a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>> index b44c487727d4..782e2700290b 100644
>> --- a/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>> +++ b/arch/x86/kernel/cpu/resctrl/ctrlmondata.c
>> @@ -87,10 +87,13 @@ int parse_bw(struct rdt_parse_data *data, struct resctrl_schema *s,
>>  
>>  /*
>>   * Check whether a cache bit mask is valid.
>> - * For Intel the SDM says:
>> - *	Please note that all (and only) contiguous '1' combinations
>> - *	are allowed (e.g. FFFFH, 0FF0H, 003CH, etc.).
>> - * Additionally Haswell requires at least two bits set.
>> + * On Intel CPUs, non-contiguous 1s value support is indicated by CPUID:
>> + *   - CPUID.0x10.1:ECX[3]: L3 non-contiguous 1s value supported if 1
>> + *   - CPUID.0x10.2:ECX[3]: L2 non-contiguous 1s value supported if 1
>> + *
>> + * Additionally Haswell requires at least two bits set. Since it does not
>> + * have CPUID enumeration support for Cache allocation the non-contiguous 1s
>> + * value support is disabled.
>
>The second sentence can be misinterpreted. non-contiguous 1s value support is
>disabled on Haswell because Haswell does not support non-contiguous 1s value,
>not because it does not have CPUID enumeration for RDT.

Would phrasing it like this:

+ * Additionally Haswell requires at least two bits set. Since it does not
+ * allow the non-contiguous 1s for cache allocation the support is
+ * hardcoded to disabled.

be more sensible?

>>   * AMD allows non-contiguous bitmasks.
>
>(connecting to comments in following patch on this topic ... notice how the above
>comments use the term "bit mask" and "bitmasks")

I'll change it so the naming is unified, thanks.

>>   */
>>  static bool cbm_validate(char *buf, u32 *data, struct rdt_resource *r)
>> diff --git a/arch/x86/kernel/cpu/resctrl/internal.h b/arch/x86/kernel/cpu/resctrl/internal.h
>> index 85ceaf9a31ac..c47ef2f13e8e 100644
>> --- a/arch/x86/kernel/cpu/resctrl/internal.h
>> +++ b/arch/x86/kernel/cpu/resctrl/internal.h
>> @@ -492,6 +492,15 @@ union cpuid_0x10_3_eax {
>>  	unsigned int full;
>>  };
>>  
>> +/* CPUID.(EAX=10H, ECX=ResID).ECX */
>> +union cpuid_0x10_x_ecx {
>> +	struct {
>> +		unsigned int reserved:3;
>> +		unsigned int noncont:1;
>> +	} split;
>> +	unsigned int full;
>> +};
>> +
>>  /* CPUID.(EAX=10H, ECX=ResID).EDX */
>>  union cpuid_0x10_x_edx {
>>  	struct {
>
>The core of the change looks good to me.
>
>Reinette

Thanks for reviewing!

-- 
Kind regards
Maciej Wieczór-Retman