linux-kernel - Re: [RFC PATCH 1/2] tracing/user_events: Use remote writes for event enablement

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Date:   Sat, 29 Oct 2022 10:44:32 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Beau Belgrave <beaub@...ux.microsoft.com>, rostedt@...dmis.org,
        mhiramat@...nel.org, dcook@...ux.microsoft.com,
        alanau@...ux.microsoft.com
Cc:     linux-trace-devel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/2] tracing/user_events: Use remote writes for event
 enablement

On 2022-10-27 18:40, Beau Belgrave wrote:

[...]
> diff --git a/include/linux/user_events.h b/include/linux/user_events.h
> index 592a3fbed98e..4c3bd16395a9 100644
> --- a/include/linux/user_events.h
> +++ b/include/linux/user_events.h
> @@ -33,12 +33,16 @@ struct user_reg {
>   	/* Input: Size of the user_reg structure being used */
>   	__u32 size;
>   
> +	/* Input: Flags/common settings */
> +	__u32 enable_bit : 5, /* Bit in enable address to use (0-31) */
> +	      __reserved : 27;

I'm always worried about using C/C++ bitfields in uapi, because some 
compilers (e.g. MS Windows Compiler) have different implementation of 
the bitfields. See

gcc(1)

-Wnopacked-bitfield-compat
-mms-bitfields / -mno-ms-bitfields

Thanks,

Mathieu

> +
> +	/* Input: Address to update when enabled */
> +	__u64 enable_addr;
> +
>   	/* Input: Pointer to string with event name, description and flags */
>   	__u64 name_args;
>   
> -	/* Output: Bitwise index of the event within the status page */
> -	__u32 status_bit;
> -
>   	/* Output: Index of the event to use when writing data */
>   	__u32 write_index;
>   } __attribute__((__packed__));
> diff --git a/kernel/trace/trace_events_user.c b/kernel/trace/trace_events_user.c-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com