On Wed, May 10, 2017 at 10:32:06AM +0200, Jiri Slaby wrote: > On 05/09/2017, 09:22 PM, Josh Poimboeuf wrote: > > On Tue, May 09, 2017 at 08:47:50PM +0200, Jiri Kosina wrote: > >> On Sun, 7 May 2017, Josh Poimboeuf wrote: > >> > >>> DWARF is great for debuggers. It helps you find all the registers on > >>> the stack, so you can see function arguments and local variables. All > >>> expressed in a nice compact format. > >>> > >>> But that's overkill for unwinders. We don't need all those registers, > >>> and the state machine is too complicated. > >> > >> OTOH if we make the failures in processing of those "auxiliary" > >> information non-fatal (in a sense that it neither causes kernel bug nor > >> does it actually corrupt the unwinding process, but the only effect is > >> losing "optional" information), having this data available doesn't hurt. > > > > But it does hurt, in the sense that the complicated format of DWARF CFI > > means the unwinder has to jump through a lot more hoops to read it. > > Why that matters, actually? Unwinder is nothing to be performance > oriented. And if somebody is doing a lot of unwinding during runtime, > they can switch to in-this-case-faster FP unwinder. More complexity == more bugs. > > And anyway, fixing the correctness of the DWARF data is only half the > > problem IMO. The other half of the problem is unwinder complexity. > > Complex, but generic and working. IMO, it would be rather though to come > up with some tool working on different compilers or even different > versions of gcc. I mean some tool to convert the DWARF data to something > proprietary. The conversion would be as complex as is the unwinder plus > conversion to the proprietary format and its dump into ELF. We would > still rely on a (now out-of-kernel-runtime-code) complex monolith to do > it right. Complexity outside of the kernel is infinitely better than complexity in mission critical oops code. -- Josh -- To unsubscribe from this list: send the line "unsubscribe live-patching" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
