netdev - Re: [PATCH bpf-next] libbpf: generate more efficient BPF_CORE

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Message-ID: <20191011212631.GE21131@pc-63.home>
Date:   Fri, 11 Oct 2019 23:26:31 +0200
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Andrii Nakryiko <andriin@...com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...com,
        andrii.nakryiko@...il.com, kernel-team@...com
Subject: Re: [PATCH bpf-next] libbpf: generate more efficient BPF_CORE_READ
 code

On Thu, Oct 10, 2019 at 07:38:47PM -0700, Andrii Nakryiko wrote:
> Existing BPF_CORE_READ() macro generates slightly suboptimal code. If
> there are intermediate pointers to be read, initial source pointer is
> going to be assigned into a temporary variable and then temporary
> variable is going to be uniformly used as a "source" pointer for all
> intermediate pointer reads. Schematically (ignoring all the type casts),
> BPF_CORE_READ(s, a, b, c) is expanded into:
> ({
> 	const void *__t = src;
> 	bpf_probe_read(&__t, sizeof(*__t), &__t->a);
> 	bpf_probe_read(&__t, sizeof(*__t), &__t->b);
> 
> 	typeof(s->a->b->c) __r;
> 	bpf_probe_read(&__r, sizeof(*__r), &__t->c);
> })
> 
> This initial `__t = src` makes calls more uniform, but causes slightly
> less optimal register usage sometimes when compiled with Clang. This can
> cascase into, e.g., more register spills.
> 
> This patch fixes this issue by generating more optimal sequence:
> ({
> 	const void *__t;
> 	bpf_probe_read(&__t, sizeof(*__t), &src->a); /* <-- src here */
> 	bpf_probe_read(&__t, sizeof(*__t), &__t->b);
> 
> 	typeof(s->a->b->c) __r;
> 	bpf_probe_read(&__r, sizeof(*__r), &__t->c);
> })
> 
> Fixes: 7db3822ab991 ("libbpf: Add BPF_CORE_READ/BPF_CORE_READ_INTO helpers")
> Signed-off-by: Andrii Nakryiko <andriin@...com>

Applied, thanks!