linux-kernel - Re: [PATCH v11 10/23] x86/resctrl: Remove MSR reading of event configuration value

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Message-ID: <b09e5829-6192-47f0-aed4-93116c33f4a0@amd.com>
Date: Tue, 11 Feb 2025 13:44:59 -0600
From: "Moger, Babu" <babu.moger@....com>
To: Xin Li <xin@...or.com>, Reinette Chatre <reinette.chatre@...el.com>,
 corbet@....net, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
 dave.hansen@...ux.intel.com, tony.luck@...el.com, peternewman@...gle.com
Cc: x86@...nel.org, hpa@...or.com, paulmck@...nel.org,
 akpm@...ux-foundation.org, thuth@...hat.com, rostedt@...dmis.org,
 xiongwei.song@...driver.com, pawan.kumar.gupta@...ux.intel.com,
 daniel.sneddon@...ux.intel.com, jpoimboe@...nel.org, perry.yuan@....com,
 sandipan.das@....com, kai.huang@...el.com, xiaoyao.li@...el.com,
 seanjc@...gle.com, xin3.li@...el.com, andrew.cooper3@...rix.com,
 ebiggers@...gle.com, mario.limonciello@....com, james.morse@....com,
 tan.shaopeng@...itsu.com, linux-doc@...r.kernel.org,
 linux-kernel@...r.kernel.org, maciej.wieczor-retman@...el.com,
 eranian@...gle.com
Subject: Re: [PATCH v11 10/23] x86/resctrl: Remove MSR reading of event
 configuration value

Hi Xin,

On 2/7/25 04:07, Xin Li wrote:
> On 2/6/2025 8:17 AM, Reinette Chatre wrote:
>>>> +    wrmsr(MSR_IA32_EVT_CFG_BASE + index, mon_info->mon_config, 0);
>>> This is the existing code, however it would be better to use wrmsrl()
>>> when the higher 32-bit are all 0s:
>>>
>>>      wrmsrl(MSR_IA32_EVT_CFG_BASE + index, mon_info->mon_config);
>>>
>> Could you please elaborate what makes this change better?
> 
> In short, it takes one less argument, and doesn't pass an argument of 0.
> 
> The longer story is that hpa and I are refactoring the MSR access APIs
> to accommodate the immediate form of MSR access instructions.  And we
> are not happy about that there are too many MSR access APIs and their
> uses are *random*.  The native wrmsr() and wrmsrl() are essentially the
> same and the only difference is that wrmsr() passes a 64-bit value to be
> written into a MSR in *2* u32 arguments.  But we already have struct msr
> defined in asm/shared/msr.h as:
>     struct msr {
>             union {
>                     struct {
>                             u32 l;
>                             u32 h;
>                     };
>                     u64 q;
>             };
>     };
> 
> it's more natural to do the same job with this data structure in most
> cases.  And we want to remove wrmsr() and only keep wrmsrl(), thus a
> developer won't have to figure out which one is better to use :-P.
> 
> For that to happen, one cleanup is to replace wrmsr(msr, low, 0) with
> wrmsrl(msr, low) (low is automatically converted to u64 from u32).
> 
> However, I'm fine if Babu wants to keep it as-is.

Thanks for the explanation.  Changed it to use wrmsrl().

-- 
Thanks
Babu Moger