On Wed, Aug 13, 2014 at 4:34 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote: > On Wed, Aug 13, 2014 at 4:25 PM, David Miller <davem@xxxxxxxxxxxxx> wrote: >> From: David Laight <David.Laight@xxxxxxxxxx> >> 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).) That would be an interesting optimization. I did movabsq just because it was straightforward. JITs can play interesting tricks here. Since it's really a constant value, there is no difference whether it's a pointer or a constant. If JIT can use $rip trick on x64 or reduce number of sethi insns on sparc, it should try to do it regardless of how value in dst_reg will be used later on by the program. JITs can also allocate some read-only area for constants and do a relative load from there. Not sure that it will be faster though. JITs can get more complex and smarter as time goes by. They can even randomly do some ld_imm64 via movabsq and some via a sequence of mov, shift, or. That will through away JIT spraying attacks. If JITed code itself is random, that would be nice defense. -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
