On Thu, Jul 11, 2019 at 4:51 AM Peter Zijlstra <[email protected]> wrote: > > Since INT3/#BP no longer runs on an IST, this workaround is no longer > required. > > Tested by running lockdep+ftrace as described in the initial commit: > > 5963e317b1e9 ("ftrace/x86: Do not change stacks in DEBUG when calling > lockdep") > > Reviewed-by: Steven Rostedt (VMware) <[email protected]> > Signed-off-by: Peter Zijlstra (Intel) <[email protected]>
I would definitely like to see this happen, but there are all kinds of possibly nasties here. Ideally we'd like get rid of IST for #DB, but we can't due to the MOV SS mess. There are a few relevant cases we care about: #DB from user mode -> anything that hits in C code: irrelevant -- we've exited the IST stack already. #DB from user mode -> NMI/MCE in the asm -> #DB: The NMI code tries to get this right. The MCE code does not. #DB from kernel mode -> NMI/MCE -> #DB: same as above. MOV SS -> #DB from entry -> #DB again: ugh. We get some protection from shift_ist. IMO we would ideally just clear DR7 in sensitive contexts. Or extend the debug_stack_set_zero(), etc hack. All that being said, the actual _DEBUG macros shouldn't matter here, I think. But I'd like to sleep on it. So not-yet-acked-by me.
