On Thursday, July 28, 2016 12:12:15 AM Rafael J. Wysocki wrote: > On Wednesday, July 27, 2016 12:59:18 PM Josh Poimboeuf wrote: > > On Wed, Jul 27, 2016 at 01:08:21AM +0200, Rafael J. Wysocki wrote: > > > On Wed, Jul 27, 2016 at 12:42 AM, Rafael J. Wysocki <r...@rjwysocki.net> > > > wrote: > > > > On Tuesday, July 26, 2016 04:53:19 PM Josh Poimboeuf wrote: > > > >> On Tue, Jul 26, 2016 at 10:15:39PM +0200, Rafael J. Wysocki wrote: > > > >> > On Tuesday, July 26, 2016 09:39:05 AM Josh Poimboeuf wrote: > > > >> > > On Tue, Jul 26, 2016 at 01:32:28PM +0200, Rafael J. Wysocki wrote: > > > >> > > > Hi, > > > >> > > > > > > >> > > > The following commit: > > > >> > > > > > > >> > > > commit 13523309495cdbd57a0d344c0d5d574987af007f > > > >> > > > Author: Josh Poimboeuf <jpoim...@redhat.com> > > > >> > > > Date: Thu Jan 21 16:49:21 2016 -0600 > > > >> > > > > > > >> > > > x86/asm/acpi: Create a stack frame in do_suspend_lowlevel() > > > >> > > > > > > >> > > > do_suspend_lowlevel() is a callable non-leaf function which > > > >> > > > doesn't > > > >> > > > honor CONFIG_FRAME_POINTER, which can result in bad stack > > > >> > > > traces. > > > >> > > > > > > >> > > > Create a stack frame for it when CONFIG_FRAME_POINTER is > > > >> > > > enabled. > > > >> > > > > > > >> > > > is reported to cause a resume-from-hibernation regression due to > > > >> > > > an attempt > > > >> > > > to execute an NX page (we've seen quite a bit of that recently). > > > >> > > > > > > >> > > > I'm asking the reporter to try 4.7, but if the problem is still > > > >> > > > there, we'll > > > >> > > > need to revert the above I'm afraid. > > > >> > > > > >> > So the bug is still there in 4.7 and it goes away after reverting > > > >> > the above > > > >> > commit. I guess I'll send a revert then. > > > >> > > > >> Hm, the code in wakeup_64.S seems quite magical, but I can't figure out > > > >> why this change causes a panic. Is it really causing the panic or is > > > >> it > > > >> uncovering some other bug? > > > > > > > > It doesn't matter really. > > > > > > > > It surely interacts with something in a really odd way, but that only > > > > means > > > > that its impact goes far beyond what was expected when it was applied. > > > > Its > > > > changelog is inadequate as a result and so on. > > > > > > > >> Maybe we should hold off on reverting until we understand the issue. > > > > > > > > Which very well may take forever. > > > > > > > > And AFAICS this is a fix for a theoretical issue and it *reliably* > > > > triggers a > > > > very practical kernel panic for this particular reporter. I'd rather > > > > live > > > > with the theoretical issue unfixed to be honest. > > > > > > Well, actually, the best part is that do_suspend_lowlevel() is not > > > even called during hibernation or resume from it. It only is called > > > during suspend-to-RAM. > > > > > > Question now is how the change made by the commit in question can > > > affect hibernation which is an unrelated code path. We know for a > > > fact that it does affect it, but how? > > > > Hm... I have a theory, but I'm not sure about it. I noticed that > > x86_acpi_enter_sleep_state(), > > I think you mean x86_acpi_suspend_lowlevel(). > > > which is involved in suspend, overwrites > > several global variables (e.g, initial_code) which are used by the CPU > > boot code in head_64.S. But surprisingly, it doesn't restore those > > variables to their original values after it resumes. > > Is the head_64.S code also used to bring up offline CPUs? > > If not, then this is not the problem, because hibernation doesn't use it > for the boot CPU anyway. > > > So if a suspend and resume were done before the hibernate, those > > variables would presumably have suspend-centric values, and the first > > time a CPU is brought up during the hibernation restore operation, it > > would jump to wakeup_long64() (the suspend resume function) instead of > > start_secondary (which is the normal CPU boot function). > > > > So, if true, that would explain why my patch triggers a bug: > > wakeup_long64() always[*] jumps to .Lresume_point, which my patch > > affected. Because of the FRAME_END, it would pop an extra value off the > > stack. So when restore_processor_state() returns, it would return to > > whatever random address is on the stack after the real RIP. Which is > > consistent with the oops from the bug. It had a bad instruction > > pointer, which looked like a stack address. > > OK, so why doesn't it break resume from suspend to RAM? wakeup_long64 is > invoked by the CPU startup code then and doesn't the FRAME_END affect > that too?
Ah, I see. wakeup_long64 will restore RSP from saved_rsp and that points to the right address already. OK Thanks, Rafael