linux-kernel - Re: [PATCH 2/5] vdso: Add comment about reason for vdso struct ordering

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Message-ID: <87jzi5p1qs.fsf@somnus>
Date: Mon, 01 Jul 2024 17:31:55 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Vincenzo Frascino <vincenzo.frascino@....com>, Andy Lutomirski
 <luto@...nel.org>, Thomas Gleixner <tglx@...utronix.de>
Cc: linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] vdso: Add comment about reason for vdso struct
 ordering

Vincenzo Frascino <vincenzo.frascino@....com> writes:

> On 01/07/2024 15:47, Anna-Maria Behnsen wrote:
>> Signed-off-by: Anna-Maria Behnsen <anna-maria@...utronix.de>
>
> nit: Can you please add something in the commit message?

Sure, I would propose the following:

The struct vdso_data is optimized for fast access to the often required
struct members. The optimization is not documented in the struct
description but it should be kept in mind, when working with the
vdso_data struct.

Add a comment to the struct description.

>
> Otherwise:
>
> Reviewed-by: Vincenzo Frascino <vincenzo.frascino@....com>

Thanks!

>> ---
>>  include/vdso/datapage.h | 4 ++++
>>  1 file changed, 4 insertions(+)
>> 
>> diff --git a/include/vdso/datapage.h b/include/vdso/datapage.h
>> index d04d394db064..7647e0946f50 100644
>> --- a/include/vdso/datapage.h
>> +++ b/include/vdso/datapage.h
>> @@ -77,6 +77,10 @@ struct vdso_timestamp {
>>   * vdso_data will be accessed by 64 bit and compat code at the same time
>>   * so we should be careful before modifying this structure.
>>   *
>> + * The ordering of the struct members is optimized to have fast access to the
>> + * often required struct members which are related to CLOCK_REALTIME and
>> + * CLOCK_MONOTONIC. This information is stored in the first cache lines.
>> + *
>>   * @basetime is used to store the base time for the system wide time getter
>>   * VVAR page.
>>   *
>>