On Wed, 2010-01-20 at 19:31 +0100, Andi Kleen wrote: > Jim Keniston <jkeni...@us.ibm.com> writes: > > > > I don't know of any such plans, but I'd be interested to read more of > > your thoughts here. As I understand it, you've suggested replacing the > > probed instruction with a jump into an instrumentation vma (the XOL > > area, or something similar). Masami has demonstrated -- through his > > djprobes enhancement to kprobes -- that this can be done for many x86 > > instructions. > > The big problem when doing this in user space is that for 64bit > it has to be within 2GB of the probed code, otherwise you would > need to rewrite the instruction to not use any rip relative addressing, > which can be rather complicated (needs registers, but the instruction > might already use them, so you would need a register allocator/spilling etc.)
I'm probably telling you stuff you already know, but... Re: jumps longer than 2GB: The following 14-byte sequence seems to work: jmpq *(%rip) .quad next_insn where next_insn is the address of the instruction to which we want to jump. We'd need this for boosting, anyway -- to jump from the XOL area back to the probed instruction stream. I think djprobes inserts a 5-byte jump at the probepoint; I don't know whether a 14-byte jump would introduce new difficulties. Re: rewriting instructions that use rip-relative addressing. We do that now. See handle_riprel_insn() in patch #2. (As far as we can tell, it works, but we'd appreciate your review of it.) > > And that 2GB can be anywhere in the address space for shared > libraries, which might well be already used. A lot of programs > need large VM areas without holes. > > Also I personally would be unconfortable to let the instruction > decoder be used by unpriviledged code. Who knows how > many buffer overflows it has? The instruction decoder is used only during instruction analysis, while registering the probe -- i.e., in kernel space. > > In general the trend has been also to make traps faster in the CPU, make > sure you're not optimizing for some old CPU here. I won't argue with that. What Avi seems to be proposing buys us a speedup, but at the cost of increased complexity -- among other things, splitting the instrumentation code between user space (in the "XOL" area -- which would then be used for much more than XOL instruction slots) and kernel space. The splitting would presumably be handled by higher-level code -- SystemTap, perf, or whatever. It's a neat idea, but it seems like a v2 kind of feature. > > -Andi Jim