bpf: Support inlining bpf_kptr_xchg() helper

The motivation of inlining bpf_kptr_xchg() comes from the performance
profiling of bpf memory allocator benchmark. The benchmark uses
bpf_kptr_xchg() to stash the allocated objects and to pop the stashed
objects for free. After inling bpf_kptr_xchg(), the performance for
object free on 8-CPUs VM increases about 2%~10%. The inline also has
downside: both the kasan and kcsan checks on the pointer will be
unavailable.

bpf_kptr_xchg() can be inlined by converting the calling of
bpf_kptr_xchg() into an atomic_xchg() instruction. But the conversion
depends on two conditions:
1) JIT backend supports atomic_xchg() on pointer-sized word
2) For the specific arch, the implementation of xchg is the same as
   atomic_xchg() on pointer-sized words.

It seems most 64-bit JIT backends satisfies these two conditions. But
as a precaution, defining a weak function bpf_jit_supports_ptr_xchg()
to state whether such conversion is safe and only supporting inline for
64-bit host.

For x86-64, it supports BPF_XCHG atomic operation and both xchg() and
atomic_xchg() use arch_xchg() to implement the exchange, so enabling the
inline of bpf_kptr_xchg() on x86-64 first.

Reviewed-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20240105104819.3916743-2-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 10 insertions, 0 deletions

diff --git a/kernel/bpf/core.c b/kernel/bpf/core.c
index ea6843be2616..fbb1d95a9b44 100644
--- a/kernel/bpf/core.c
+++ b/kernel/bpf/core.c
@@ -2925,6 +2925,16 @@ bool __weak bpf_jit_supports_far_kfunc_call(void)
 	return false;
 }
 
+/* Return TRUE if the JIT backend satisfies the following two conditions:
+ * 1) JIT backend supports atomic_xchg() on pointer-sized words.
+ * 2) Under the specific arch, the implementation of xchg() is the same
+ *    as atomic_xchg() on pointer-sized words.
+ */
+bool __weak bpf_jit_supports_ptr_xchg(void)
+{
+	return false;
+}
+
 /* To execute LD_ABS/LD_IND instructions __bpf_prog_run() may call
  * skb_copy_bits(), so provide a weak definition of it for NET-less config.
  */