On Mon, Aug 15, 2022 at 7:13 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote: > > On Wed, 11 Dec 2019 13:30:13 +0100 > Björn Töpel <bjorn.topel@xxxxxxxxx> wrote: > > > From: Björn Töpel <bjorn.topel@xxxxxxxxx> > > > > The BPF dispatcher is a multi-way branch code generator, mainly > > targeted for XDP programs. When an XDP program is executed via the > > bpf_prog_run_xdp(), it is invoked via an indirect call. The indirect > > call has a substantial performance impact, when retpolines are > > enabled. The dispatcher transform indirect calls to direct calls, and > > therefore avoids the retpoline. The dispatcher is generated using the > > BPF JIT, and relies on text poking provided by bpf_arch_text_poke(). > > > > The dispatcher hijacks a trampoline function it via the __fentry__ nop > > Why was the ftrace maintainers not Cc'd on this patch? I would have NACKED > it. Hell, it wasn't even sent to LKML! This was BPF being sneaky in > updating major infrastructure of the Linux kernel without letting the > stakeholders of this change know about it. > > For some reason, the BPF folks think they own the entire kernel! > > When I heard that ftrace was broken by BPF I thought it was something > unique they were doing, but unfortunately, I didn't investigate what they > were doing at the time. ftrace is still broken and refusing to accept the fact doesn't make it non-broken. > Then they started sending me patches to hide fentry locations from ftrace. > And even telling me that fentry != ftrace It sounds that you've invented nop5 and kernel's ability to replace nop5 with a jump or call. ftrace should really stop trying to own all of the kernel text rewrites. It's in the way. Like this case. > https://lore.kernel.org/all/CAADnVQJTT7h3MniVqdBEU=eLwvJhEKNLSjbUAK4sOrhN=zggCQ@xxxxxxxxxxxxxx/ > > Even though fentry was created for ftrace > > https://lore.kernel.org/lkml/1258720459.22249.1018.camel@xxxxxxxxxxxxxxxxxxx/ > > and all the work with fentry was for the ftrace infrastructure. Ftrace > takes a lot of care for security and use cases for other users (like > live kernel patching). But BPF has the NIH syndrome, and likes to own > everything and recreate the wheel so that they have full control. > > > of the trampoline. One dispatcher instance currently supports up to 64 > > dispatch points. A user creates a dispatcher with its corresponding > > trampoline with the DEFINE_BPF_DISPATCHER macro. > > Anyway, this patch just looks like a re-implementation of static_calls: It was implemented long before static_calls made it to the kernel and it's different. Please do your home work.
