On Wed, Jan 30, 2019 at 10:36:18AM -0800, Paul E. McKenney wrote: > On Wed, Jan 30, 2019 at 06:11:00PM +0000, Will Deacon wrote: > > Hi Alexei, > > > > On Mon, Jan 28, 2019 at 01:56:24PM -0800, Alexei Starovoitov wrote: > > > On Mon, Jan 28, 2019 at 10:24:08AM +0100, Peter Zijlstra wrote: > > > > On Fri, Jan 25, 2019 at 04:17:26PM -0800, Alexei Starovoitov wrote: > > > > > What I want to avoid is to define the whole execution ordering model > > > > > upfront. > > > > > We cannot say that BPF ISA is weakly ordered like alpha. > > > > > Most of the bpf progs are written and running on x86. We shouldn't > > > > > twist bpf developer's arm by artificially relaxing memory model. > > > > > BPF memory model is equal to memory model of underlying architecture. > > > > > What we can do is to make it bpf progs a bit more portable with > > > > > smp_rmb instructions, but we must not force weak execution on the > > > > > developer. > > > > > > > > Well, I agree with only introducing bits you actually need, and my > > > > smp_rmb() example might have been poorly chosen, smp_load_acquire() / > > > > smp_store_release() might have been a far more useful example. > > > > > > > > But I disagree with the last part; we have to pick a model now; > > > > otherwise you'll pain yourself into a corner. > > > > > > > > Also; Alpha isn't very relevant these days; however ARM64 does seem to > > > > be gaining a lot of attention and that is very much a weak architecture. > > > > Adding strongly ordered assumptions to BPF now, will penalize them in > > > > the long run. > > > > > > arm64 is gaining attention just like riscV is gaining it too. > > > BPF jit for arm64 is very solid, while BPF jit for riscV is being worked > > > on. > > > BPF is not picking sides in CPU HW and ISA battles. > > > > It's not about picking a side, it's about providing an abstraction of the > > various CPU architectures out there so that the programmer doesn't need to > > worry about where their program may run. Hell, even if you just said "eBPF > > follows x86 semantics" that would be better than saying nothing (and then we > > could have a discussion about whether x86 semantics are really what you > > want). > > To reinforce this point, the Linux-kernel memory model (tools/memory-model) > is that abstraction for the Linux kernel. Why not just use that for BPF?
I already answered this earlier in the thread. tldr: not going to sacrifice performance.
