linux-kernel - Re: [PATCH v3 10/11] trace: platform/x86/intel/ifs: Add trace point to track Intel IFS operations

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Message-ID: <20220421084145.3b0c3539@gandalf.local.home>
Date:   Thu, 21 Apr 2022 08:41:45 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     "hdegoede@...hat.com" <hdegoede@...hat.com>,
        "markgross@...nel.org" <markgross@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "x86@...nel.org" <x86@...nel.org>, "hpa@...or.com" <hpa@...or.com>,
        "corbet@....net" <corbet@....net>,
        "gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
        "andriy.shevchenko@...ux.intel.com" 
        <andriy.shevchenko@...ux.intel.com>,
        "Joseph, Jithu" <jithu.joseph@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "platform-driver-x86@...r.kernel.org" 
        <platform-driver-x86@...r.kernel.org>,
        "patches@...ts.linux.dev" <patches@...ts.linux.dev>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>
Subject: Re: [PATCH v3 10/11] trace: platform/x86/intel/ifs: Add trace point
 to track Intel IFS operations

On Thu, 21 Apr 2022 04:26:39 +0000
"Luck, Tony" <tony.luck@...el.com> wrote:

> >> +TRACE_EVENT(ifs_status,
> >> +
> >> +	TP_PROTO(union ifs_scan activate, union ifs_status status),  
> >
> > Really, you want to pass the structure in by value, so that we have two
> > copies? One to get to this function and then one to write to the ring
> > buffer?  
> 
> These "structures" are just bitfield helpers for a u64 that is passed into
> WRMSR (in the case of activate) and received back from RDMSR in
> the case of status.
> 
> So this is really just a pair of u64 arguments, with the compiler handling
> the bit field extractions into the ring buffer.

I was just wondering if passing by reference would be better, but as you
stated, they are just two u64 arguments.

> 
> Here are the definitions:
> 
> union ifs_scan {
>         u64     data;
>         struct {
>                 u32     start   :8;
>                 u32     stop    :8;
>                 u32     rsvd    :16;
>                 u32     delay   :31;
>                 u32     sigmce  :1;
>         };
> };
> 
> union ifs_status {
>         u64     data;
>         struct {
>                 u32     chunk_num               :8;
>                 u32     chunk_stop_index        :8;
>                 u32     rsvd1                   :16;
>                 u32     error_code              :8;
>                 u32     rsvd2                   :22;
>                 u32     control_error           :1;
>                 u32     signature_error         :1;
>         };
> };
> 
> Would it be better to do the bit extractions of the start/stop fields first?

No, I'm just paranoid about passing structures / unions in by value and not
reference. If you are fine with this, then I'm OK too.

-- Steve