On Fri Mar 1, 2024 at 7:45 PM AEST, Thomas Huth wrote: > On 27/02/2024 09.50, Thomas Huth wrote: > > On 26/02/2024 11.11, Nicholas Piggin wrote: > >> The backtrace handler terminates when it sees a NULL caller address, > >> but the powerpc stack setup does not keep such a NULL caller frame > >> at the start of the stack. > >> > >> This happens to work on pseries because the memory at 0 is mapped and > >> it contains 0 at the location of the return address pointer if it > >> were a stack frame. But this is fragile, and does not work with powernv > >> where address 0 contains firmware instructions. > >> > >> Use the existing dummy frame on stack as the NULL caller, and create a > >> new frame on stack for the entry code. > >> > >> Signed-off-by: Nicholas Piggin <npiggin@xxxxxxxxx> > >> --- > >> powerpc/cstart64.S | 12 ++++++++++-- > >> 1 file changed, 10 insertions(+), 2 deletions(-) > > > > Thanks for tackling this! ... however, not doing powerpc work since years > > anymore, I have some ignorant questions below... > > > >> diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S > >> index e18ae9a22..14ab0c6c8 100644 > >> --- a/powerpc/cstart64.S > >> +++ b/powerpc/cstart64.S > >> @@ -46,8 +46,16 @@ start: > >> add r1, r1, r31 > >> add r2, r2, r31 > >> + /* Zero backpointers in initial stack frame so backtrace() stops */ > >> + li r0,0 > >> + std r0,0(r1) > > > > 0(r1) is the back chain pointer ... > > > >> + std r0,16(r1) > > > > ... but what is 16(r1) ? I suppose that should be the "LR save word" ? But > > isn't that at 8(r1) instead?? (not sure whether I'm looking at the right ELF > > abi spec right now...) > > Ok, I was looking at the wrong ELF spec, indeed (it was an ancient 32-bit > spec, not the 64-bit ABI). Sorry for the confusion. Having a proper #define > or a comment for the 16 here would still be helpful, though. Thanks for the deailed reviews as always. I've been a little busy with QEMU so may not get another series out for a bit. I'll probably wait for Andrew's stack backtrace changes to land too before resend. But, yes a comment makes sense. I'll add. Thanks, Nick
