On Wed, May 13, 2020 at 11:48:56PM +0200, Pavel Machek wrote: > On Wed 2020-05-13 11:45:03, Greg Kroah-Hartman wrote: > > From: Josh Poimboeuf <jpoimboe@xxxxxxxxxx> > > > > commit 06a9750edcffa808494d56da939085c35904e618 upstream. > > > > The PUSH_AND_CLEAR_REGS macro zeroes each register immediately after > > pushing it. If an NMI or exception hits after a register is cleared, > > but before the UNWIND_HINT_REGS annotation, the ORC unwinder will > > wrongly think the previous value of the register was zero. This can > > confuse the unwinding process and cause it to exit early. > > > > Because ORC is simpler than DWARF, there are a limited number of unwind > > annotation states, so it's not possible to add an individual unwind hint > > after each push/clear combination. Instead, the register clearing > > instructions need to be consolidated and moved to after the > > UNWIND_HINT_REGS annotation. > > This actually makes kernel entry/exit slower, due to poor instruction > scheduling. And that is a bit of hot path... Is it strictly > neccessary? Not everyone needs ORC scheduler. Should it be somehow > optional? I didn't measure a difference beyond the noise level, did you? -- Josh
