On 14.11.2016 16:11, John David Anglin wrote: > On 2016-11-14 3:21 AM, Jeff Law wrote: >> On 11/13/2016 12:48 PM, Helge Deller wrote: >>> On 13.11.2016 19:56, Jeff Law wrote: >>>> On 11/13/2016 11:37 AM, Helge Deller wrote: >>>>> If you are going to change the ABI, maybe we can add more >>>>> things as well? Which comes to my mind here is for example an >>>>> optimized mcount() function which allows changing the return >>>>> pointer (see -mmcount-ra-address on MIPS) ? >>> >>>> As in twiddling RP to return to a different point? >>> >>> No, that's not the use case for me. I was wrong. It's actually the use case to modify the RP... >>> I was working on the ftrace functionality in the Linux kernel. >>> I'd need to look up the full details again, but as far as I >>> remember one of the tracers wants to know the function to which >>> the caller of mcount() would return, so some kind of simple >>> __builtin_return_address(2). >> Ah. Isn't that going to be sitting at sp-20 or something like >> that. My PA is rusty, but my recollection is that's supposed to be >> at a fixed location in the frame. Yes, sp-10. 0000000000000000 <irq_to_desc>: 0: 08 03 02 41 copy r3,r1 4: 0f c2 12 c1 std rp,-10(sp) 8: 08 1e 02 43 copy sp,r3 c: 73 c1 00 a8 std,ma r1,50(sp) 10: 37 dd 00 20 ldo 10(sp),ret1 14: 0c 65 12 d0 std r5,8(r3) 18: 00 00 14 b9 mfia r25 1c: db 45 0b e0 extrd,u,* r26,63,32,r5 20: 70 64 00 20 std r4,10(r3) 24: 08 02 02 5a copy rp,r26 28: 37 39 3f d1 ldo -18(r25),r25 2c: 08 1b 02 44 copy dp,r4 30: 2b 60 00 00 addil L%0,dp,r1 34: 50 21 00 00 ldd 0(r1),r1 38: 0c 20 10 c1 ldd 0(r1),r1 3c: 50 22 00 20 ldd 10(r1),rp 40: e8 40 f0 00 bve,l (rp),rp 44: 50 3b 00 30 ldd 18(r1),dp > The return address of the the function to which the caller of > mcount() would return is passed to mcount() in %r26. The saved value > in the frame is not directly useful as one lacks the frame offset of > the routine calling mcount. What I want to archieve is to modify the return pointer, in order to be able to track when the function returns to his caller. The kernel ftracer uses this then to generate call stacks and to time the function. Looking at the above code, it should then be possible for me to modify -10(r3), but is there a guarantee that it's always at -10(r3) and that r3 is used? That's the reason I asked if we could modify mcount to give the address (in the stack) of the return pointer, but maybe it's just overkill for this use case ? Helge -- To unsubscribe from this list: send the line "unsubscribe linux-parisc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
