netdev - Re: [PATCH v3 bpf-next 01/10] treewide: remove struct-pass-by-value from tracepoints arguments

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Message-ID: <6b1952c4-1612-7abb-49c6-9bbaf6dc6997@fb.com>
Date:   Thu, 22 Mar 2018 12:31:12 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     <davem@...emloft.net>, <daniel@...earbox.net>,
        <torvalds@...ux-foundation.org>, <peterz@...radead.org>,
        <netdev@...r.kernel.org>, <kernel-team@...com>,
        <linux-api@...r.kernel.org>
Subject: Re: [PATCH v3 bpf-next 01/10] treewide: remove struct-pass-by-value
 from tracepoints arguments

On 3/22/18 11:11 AM, Steven Rostedt wrote:
> On Thu, 22 Mar 2018 11:01:48 -0700
> Alexei Starovoitov <ast@...com> wrote:
>
>> From: Alexei Starovoitov <ast@...nel.org>
>>
>> Fix all tracepoint arguments to pass structures (large and small) by reference
>> instead of by value.
>> Avoiding passing large structs by value is a good coding style.
>> Passing small structs sometimes is beneficial, but in all cases
>> it makes no difference vs readability of the code.
>> The subsequent patch enforces that all tracepoints args are either integers
>> or pointers and fit into 64-bit.
>
> But some of these structures are used to force type checking, and are
> just the same size as a number. That's why they don't have "struct" in
> front of them. Like pmd_t. Will the subsequent patches really break if
> the structure itself has one element that is of size long? Just seems
> to add extra code to pass in an address to something that fits into a
> single register.

yeah. C doesn't allow casting of 'struct s { u64 var };' into u64
without massive hacks and aliasing warnings by compiler.
CAST_TO_U64 macro in patch 7 will prevent tracepoint arguments to be
things like pmd_t. It's not perfect, but doing & of pmd_t variable
is imo clean enough as you can see in this patch.
The macro can be tweaked to do the cast like
*(sizeof(typeof(arg))*)&arg,
but there is no way to get rid of compiler warning.