Commit f760d053 authored by Andrii Nakryiko's avatar Andrii Nakryiko Committed by Daniel Borkmann
Browse files

libbpf: Provide barrier() and barrier_var() in bpf_helpers.h 



Add barrier() and barrier_var() macros into bpf_helpers.h to be used by
end users. While a bit advanced and specialized instruments, they are
sometimes indispensable. Instead of requiring each user to figure out
exact asm volatile incantations for themselves, provide them from
bpf_helpers.h.

Also remove conflicting definitions from selftests. Some tests rely on
barrier_var() definition being nothing, those will still work as libbpf
does the #ifndef/#endif guarding for barrier() and barrier_var(),
allowing users to redefine them, if necessary.

Signed-off-by: default avatarAndrii Nakryiko <andrii@kernel.org>
Signed-off-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220509004148.1801791-8-andrii@kernel.org
parent 785c3342

tools/lib/bpf/bpf_helpers.h

+24 −0
Original line number Diff line number Diff line
@@ -75,6 +75,30 @@ 
	})

#endif



/*

 * Compiler (optimization) barrier.

 */

#ifndef barrier

#define barrier() asm volatile("" ::: "memory")

#endif



/* Variable-specific compiler (optimization) barrier. It's a no-op which makes

 * compiler believe that there is some black box modification of a given

 * variable and thus prevents compiler from making extra assumption about its

 * value and potential simplifications and optimizations on this variable.

 *

 * E.g., compiler might often delay or even omit 32-bit to 64-bit casting of

 * a variable, making some code patterns unverifiable. Putting barrier_var()

 * in place will ensure that cast is performed before the barrier_var()

 * invocation, because compiler has to pessimistically assume that embedded

 * asm section might perform some extra operations on that variable.

 *

 * This is a variable-specific variant of more global barrier().

 */

#ifndef barrier_var

#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))

#endif



/*

 * Helper macro to throw a compilation error if __bpf_unreachable() gets

 * built into the resulting code. This works given BPF back end does not

tools/testing/selftests/bpf/progs/exhandler_kern.c

+0 −2
Original line number Diff line number Diff line
@@ -7,8 +7,6 @@ 
#include <bpf/bpf_tracing.h>

#include <bpf/bpf_core_read.h>



#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))



char _license[] SEC("license") = "GPL";



unsigned int exception_triggered;

tools/testing/selftests/bpf/progs/loop5.c

+0 −1
Original line number Diff line number Diff line
@@ -2,7 +2,6 @@ 
// Copyright (c) 2019 Facebook

#include <linux/bpf.h>

#include <bpf/bpf_helpers.h>

#define barrier() __asm__ __volatile__("": : :"memory")



char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/progs/profiler1.c

+0 −1
Original line number Diff line number Diff line 
// SPDX-License-Identifier: GPL-2.0

/* Copyright (c) 2020 Facebook */

#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))

#define UNROLL

#define INLINE __always_inline

#include "profiler.inc.h"

tools/testing/selftests/bpf/progs/pyperf.h

+0 −2
Original line number Diff line number Diff line
@@ -171,8 +171,6 @@ struct process_frame_ctx { 
	bool done;

};



#define barrier_var(var) asm volatile("" : "=r"(var) : "0"(var))



static int process_frame_callback(__u32 i, struct process_frame_ctx *ctx)

{

	int zero = 0;