On Tue, May 26, 2020 at 08:54:19PM +0100, Will Deacon wrote: > On Tue, May 26, 2020 at 11:26:11AM +0100, Mark Rutland wrote: > > On Mon, May 11, 2020 at 10:52:07AM +0800, Jiping Ma wrote: > > > Modified the patch subject and the change description. > > > > > > PC value is get from regs[15] in REGS_ABI_32 mode, but correct PC > > > is regs->pc(regs[PERF_REG_ARM64_PC]) in arm64 kernel, which caused > > > that perf can not parser the backtrace of app with dwarf mode in the > > > 32bit system and 64bit kernel. > > > > > > Signed-off-by: Jiping Ma <jiping....@windriver.com> > > > > Thanks for this. > > > > > > > --- > > > arch/arm64/kernel/perf_regs.c | 4 ++++ > > > 1 file changed, 4 insertions(+) > > > > > > diff --git a/arch/arm64/kernel/perf_regs.c b/arch/arm64/kernel/perf_regs.c > > > index 0bbac61..0ef2880 100644 > > > --- a/arch/arm64/kernel/perf_regs.c > > > +++ b/arch/arm64/kernel/perf_regs.c > > > @@ -32,6 +32,10 @@ u64 perf_reg_value(struct pt_regs *regs, int idx) > > > if ((u32)idx == PERF_REG_ARM64_PC) > > > return regs->pc; > > > > > > + if (perf_reg_abi(current) == PERF_SAMPLE_REGS_ABI_32 > > > + && idx == 15) > > > + return regs->pc; > > > > I think there are some more issues here, and we may need a more > > substantial rework. For a compat thread, we always expose > > PERF_SAMPLE_REGS_ABI_32 via per_reg_abi(), but for some reason > > perf_reg_value() also munges the compat SP/LR into their ARM64 > > equivalents, which don't exist in the 32-bit sample ABI. We also don't > > zero the regs that don't exist in 32-bit (including the aliasing PC). > > I think this was for the case where you have a 64-bit perf profiling a > 32-bit task, and it was passing the registers off to libunwind. Won't that > break if we follow your suggestion?
Oh yuck; have we messed up the ABI here, or have I misunderstood? Is arm64's PERF_SAMPLE_REGS_ABI_32 supposed to be the same as the 32-bit arm's PERF_SAMPLE_REGS_ABI_32? If yes, and the differences are being relied upon by 64-bit consumers, that's a nasty ABI issue we've introduced for compat tasks, and I don't think this patch alone is quite right. If no, then I don't see that any change is necessary, as we already expose the information, and it's a userspace bug to expect the PC in a place where the kernel has never exposed it. Thanks, Mark.
