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Date: Wed, 13 Aug 2014 16:34:59 -0700 From: Andy Lutomirski <luto@...capital.net> To: David Miller <davem@...emloft.net> Cc: David Laight <David.Laight@...lab.com>, Alexei Starovoitov <ast@...mgrid.com>, Ingo Molnar <mingo@...nel.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Steven Rostedt <rostedt@...dmis.org>, Daniel Borkmann <dborkman@...hat.com>, Chema Gonzalez <chema@...gle.com>, Eric Dumazet <edumazet@...gle.com>, Peter Zijlstra <a.p.zijlstra@...llo.nl>, "H. Peter Anvin" <hpa@...or.com>, Andrew Morton <akpm@...ux-foundation.org>, Kees Cook <keescook@...omium.org>, Linux API <linux-api@...r.kernel.org>, Network Development <netdev@...r.kernel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org> Subject: Re: [PATCH RFC v4 net-next 00/26] BPF syscall, maps, verifier, samples, llvm On Wed, Aug 13, 2014 at 4:25 PM, David Miller <davem@...emloft.net> wrote: > From: David Laight <David.Laight@...LAB.COM> > Date: Wed, 13 Aug 2014 08:52:30 +0000 > >> From: Of Alexei Starovoitov >>> one more RFC... >>> >>> Major difference vs previous set is a new 'load 64-bit immediate' eBPF insn. >>> Which is first 16-byte instruction. It shows how eBPF ISA can be extended >>> while maintaining backward compatibility, but mainly it cleans up eBPF >>> program access to maps and improves run-time performance. >> >> Wouldn't it be more sensible to follow the scheme used by a lot of cpus >> and add a 'load high' instruction (follow with 'add' or 'or'). >> It still takes 16 bytes to load a 64bit immediate value, but the instruction >> size remains constant. >> There is nothing to stop any JIT software detecting the instruction pair. > > The opposite argument is that JITs can expand the IMM64 load into whatever > sequence of instructions is most optimal. > > My only real gripe with IMM64 loads is that it's not mainly for > loading an immediate, it's for loading a pointer. And this > distinction is important for some JITs. > > For example, on sparc64 all symbol based addresses are actually 32-bit > because of the code model we use to compile the kernel and all modules. > So if we knew this is a pointer load and it's to a symbol in a kernel > or module image, we could do a 32-bit load. This is true for x86_64 as well, I think. (Almost. For x86_64 we have a choice between a sign-extended load of a value in the top 2GB of the address space and lea reg,offset(%rip).) --Andy -- To unsubscribe from this list: send the line "unsubscribe netdev" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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