netdev - Re: [PATCH 1/6] perf: Add new type PERF_TYPE

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Message-ID: <20171123100214.qm3s4h5b6i4r2iaa@hirez.programming.kicks-ass.net>
Date:   Thu, 23 Nov 2017 11:02:14 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Song Liu <songliubraving@...com>
Cc:     rostedt@...dmis.org, mingo@...hat.com, davem@...emloft.net,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        daniel@...earbox.net, kernel-team@...com
Subject: Re: [PATCH 1/6] perf: Add new type PERF_TYPE_PROBE

On Wed, Nov 15, 2017 at 09:23:33AM -0800, Song Liu wrote:

> Note: We use type __u64 for pointer probe_desc instead of __aligned_u64.
> The reason here is to avoid changing the size of struct perf_event_attr,
> and breaking new-kernel-old-utility scenario. To avoid alignment problem
> with the pointer, we will (in the following patches) copy probe_desc to
> __aligned_u64 before using it as pointer.

ISTR there are only relatively few architectures where __u64 and
__aligned_u64 are not the same thing.

The comment that goes with it seems to suggest i386 has short alignment
for u64 but my compiler says differently:

        printf("%d, %d\n", sizeof(unsigned long long), __alignof__(unsigned long long));

$ gcc -m32 -o align align.c && ./align
8, 8