On Fri, Oct 20, 2017 at 03:44:12PM +0200, Torsten Duwe wrote: > On Fri, Oct 20, 2017 at 08:24:32AM -0500, Josh Poimboeuf wrote: > > On Fri, Oct 20, 2017 at 02:44:32PM +0200, Torsten Duwe wrote: > > > I have a bad feeling about the IPA stuff in general. An obj-based approach > > > is cool in a way that it still works, and is sure to work, if the IPA > > > assumptions that led to the optimisations still hold, but as soon as they > > > break, you're screwed big time. > > > > Huh? The obj-based approach (e.g., kpatch, bin-diff) inherently detects > > such changes. Or am I misunderstanding? If so, please elaborate. > > If e.g. the old callee does not use a caller-saved reg, neither does the new > code, there is no reason to actually save it. IPA will detect this and spare > the reg save/restore. If now by any coincidence the new function needs that > register, what can you do? Patch all callers as well? You'll likely end up > with a far-too-big live patch or by coding something in assembler :-( Yes, you do have to patch the caller in that case. But kpatch-build detects that the caller changed as well, so both functions get patched and it's not a problem. And I don't remember ever seeing more than one caller for such a case. At least it's never been a problem so far. -- 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
