[PATCH] arm64: kprobes: Use BRK instead of single-step when executing instructions out-of-line

[Jean-Philippe Brucker]

On Tue, Nov 03, 2020 at 09:23:16AM +0000, Will Deacon wrote:
> Yes, let's just set all of DAIF during the trampoline. Also, while I've got
> you, If you get a chance, I'd appreciate any feedback on my proposal for
> reworking our debug exception handling altogether:
> 
> https://lore.kernel.org/r/20200626095551.GA9312@willie-the-truck

Well, I stared at this for a while... It looks fine to me, but I don't
have a full picture of the trap infrastructure (not sure whether you were
asking me). I could help with testing if you get around to reworking it.

> On taking an interrupt from EL1, stash MDSCR_EL1.SS in a pcpu variable and
> clear the register bit if it was set. Then unmask only D and leave I set. On
> return from the exception, set D and restore MDSCR_EL1.SS. If we decide to
> reschedule, unmask D (i.e. we only step into interrupts if we need a
> reschedule. Alternatively, we could skip the reschedule if we were
> stepping.)

Any specific reason to treat reschedule differently, or just to keep
things simple?  I'm asking because that could be a problem with the
current code: when taking an interrupt while stepping EL1, we keep
MDSCR_EL1.SS set and unmask D before calling the IRQ handler. The step
exception might only be taken after the next context synchronization event
(on QEMU it happens at an isb in the timer handler). If the IRQ handler
doesn't happen to do any context synchronization and we reschedule, I
guess the step exception could happen after the next eret?

Thanks,
Jean