From mboxrd@z Thu Jan 1 00:00:00 1970 From: alexei.starovoitov@gmail.com (Alexei Starovoitov) Date: Wed, 11 Nov 2015 09:27:00 -0800 Subject: [PATCH 2/2] arm64: bpf: add BPF XADD instruction In-Reply-To: <20151111162341.GN9562@arm.com> References: <4902833.k8y8bz0YLV@wuerfel> <20151111102406.GB9562@arm.com> <56431B83.5060500@iogearbox.net> <20151111115851.GE9562@arm.com> <564332B0.2090103@iogearbox.net> <20151111123831.GJ9562@arm.com> <20151111125807.GP17308@twins.programming.kicks-ass.net> <56436420.9090401@iogearbox.net> <20151111162341.GN9562@arm.com> Message-ID: <20151111172659.GA86334@ast-mbp.thefacebook.com> To: linux-arm-kernel@lists.infradead.org List-Id: linux-arm-kernel.lists.infradead.org On Wed, Nov 11, 2015 at 04:23:41PM +0000, Will Deacon wrote: > > If we're going to document it, a bug tracker might be a good place to > start. The behaviour, as it stands, is broken wrt the definition of the > __sync primitives. That is, there is no way to build __sync_fetch_and_add > out of BPF_XADD without changing its semantics. BPF_XADD == atomic_add() in kernel. period. we are not going to deprecate it or introduce something else. Semantics of __sync* or atomic in C standard and/or gcc/llvm has nothing to do with this. arm64 JIT needs to JIT bpf_xadd insn equivalent to the code of atomic_add() which is 'stadd' in armv8.1. The cpu check can be done by jit and for older cpus just fall back to interpreter. trivial. > We could fix this by either: > > (1) Defining BPF_XADD to match __sync_fetch_and_add (including memory > barriers). nope. > (2) Introducing some new BPF_ atomics, that map to something like the > C11 __atomic builtins and deprecating BPF_XADD in favour of these. nope. > (3) Introducing new source-language intrinsics to match what BPF can do > (unlikely to be popular). llvm's __sync intrinsic is used temporarily until we have time to do new intrinsic in llvm that matches kernel's atomic_add() properly. It will be done similar to llvm-bpf load_byte/word intrinsics. Note that we've been hiding it under lock_xadd() wrapper, like here: https://github.com/iovisor/bcc/blob/master/examples/networking/tunnel_monitor/monitor.c#L130