On Tue, Jan 28, 2020 at 04:40:46PM +0100, Petr Mladek wrote: > On Tue 2020-01-28 09:00:14, Josh Poimboeuf wrote: > > On Tue, Jan 28, 2020 at 10:28:07AM +0100, Miroslav Benes wrote: > > > I don't think we have something special at SUSE not generally available... > > > > > > ...and I don't think it is really important to discuss that and replying > > > to the above, because there is a legitimate use case which relies on the > > > flag. We decided to support different use cases right at the beginning. > > > > > > I understand it currently complicates things for objtool, but objtool is > > > sensitive to GCC code generation by definition. "Issues" appear with every > > > new GCC version. I see no difference here and luckily it is not so > > > difficult to fix it. > > > > > > I am happy to help with acting on those objtool warning reports you > > > mentioned in the other email. Just Cc me where appropriate. We will take a > > > look. > > > > As I said, the objtool warnings aren't even the main issue. > > Great. > > Anyway, I think that we might make your life easier with using > the proposed -Wsuggest-attribute=noreturn. Maybe. Though if I understand correctly, this doesn't help for any of the new warnings because they're for static functions, and this only warns about global functions. > Also it might be possible to create the list of global > noreturn functions using some gcc tool. Similar way that we get > the list of functions that need to be livepatched explicitly > because of the problematic optimizations. > > It sounds like a win-win approach. I don't quite get how that could be done in an automated way, but ideas about how to implement it would certainly be welcome. > > There are N users[*] of CONFIG_LIVEPATCH, where N is perhaps dozens. > > For N-1 users, they have to suffer ALL the drawbacks, with NONE of the > > benefits. > > You wrote in the other mail: > > > The problems associated with it: performance, LTO incompatibility, > > clang incompatibility (I think?), the GCC dead code issue. > > SUSE performance team did extensive testing and did not found > any real performance issues. It was discussed when the option > was enabled upstream. > > Are the other problems affecting real life usage, please? > Could you be more specific about them, please? The original commit mentioned 1-3% scheduler degradation. And I'd expect things to worsen over time as interprocedural optimizations improve. Also, LTO is coming whether we like it or not. As is Clang. Those are real-world things which will need to work with livepatching sooner or later. > > And, even if they wanted those benefits, they have no idea how to get > > them because the patch creation process isn't documented. > > I do not understand this. All the sample modules and selftests are > using source based livepatches. We're talking in circles. Have you read the thread? The samples are a (dangerous) joke. With or without -flive-patching. > It is actually the only somehow documented way. Sure, the > documentation might get improved. Patches are welcome. Are you suggesting for *me* to send documentation for how *you* build patches? > The option is not currently needed by the selftests only because there > is no selftest for this type of problems. But the problems are real. > They would actually deserve selftests. Again, patches are welcome. > > My understanding is that the source based livepatches is the future. I think that still remains to be seen. > N-1 users are just waiting until the 1 user develops more helper tools > for this. No. N-1 users have no idea how to make (safe) source-based patches in the first place. And if *you* don't need the tools, why would anyone else? Why not document the process and encourage the existence of other users so they can get involved and help with the tooling? > I would really like to hear about some serious problems > before we do this step back in upstream. Sometimes you need to take 1 step back before you can take 2 steps forward. I regret ACKing the original patch. It was too early. -- Josh
