Re: [RESEND PATCH] arm64: kgdb: fix single stepping

From: AKASHI Takahiro
Date: Fri Sep 16 2016 - 00:25:59 EST

Hi Jason,

Welcome back to mainline development.
I've been looking forward to your comments.

On Thu, Sep 15, 2016 at 08:04:57AM -0500, Jason Wessel wrote:
> On 04/20/2015 08:13 PM, AKASHI Takahiro wrote:
> >Jason,
> >
> >Could you please review my patch below?
> >See also arm64 maintainer's comment:
> >
> >
> >Thanks,
> >-Takahiro AKASHI
> >
> >I tried to verify kgdb in vanilla kernel on fast model, but it seems that
> >the single stepping with kgdb doesn't work correctly since its first
> >appearance at v3.15.
> >
> >On v3.15, 'stepi' command after breaking the kernel at some breakpoint
> >steps forward to the next instruction, but the succeeding 'stepi' never
> >goes beyond that.
> >On v3.16, 'stepi' moves forward and stops at the next instruction just
> >after enable_dbg in el1_dbg, and never goes beyond that. This variance of
> >behavior seems to come in with the following patch in v3.16:
> >
> > commit 2a2830703a23 ("arm64: debug: avoid accessing mdscr_el1 on fault
> > paths where possible")
> >
> >This patch
> >(1) moves kgdb_disable_single_step() from 'c' command handling to single
> > step handler.
> > This makes sure that single stepping gets effective at every 's' command.
> > Please note that, under the current implementation, single step bit in
> > spsr, which is cleared by the first single stepping, will not be set
> > again for the consecutive 's' commands because single step bit in mdscr
> > is still kept on (that is, kernel_active_single_step() in
> > kgdb_arch_handle_exception() is true).
> >(2) re-implements kgdb_roundup_cpus() because the current implementation
> > enabled interrupts naively. See below.
> >(3) removes 'enable_dbg' in el1_dbg.
> > Single step bit in mdscr is turned on in do_handle_exception()->
> > kgdb_handle_expection() before returning to debugged context, and if
> > debug exception is enabled in el1_dbg, we will see unexpected single-
> > stepping in el1_dbg.
> > Since v3.18, the following patch does the same:
> > commit 1059c6bf8534 ("arm64: debug: don't re-enable debug exceptions
> > on return from el1_dbg)
> >(4) masks interrupts while single-stepping one instruction.
> > If an interrupt is caught during processing a single-stepping, debug
> > exception is unintentionally enabled by el1_irq's 'enable_dbg' before
> > returning to debugged context.
> > Thus, like in (2), we will see unexpected single-stepping in el1_irq.
> >
> >Basically (1) and (2) are for v3.15, (3) and (4) for v3.1[67].
> >
> >@@ -176,18 +183,14 @@ int kgdb_arch_handle_exception(int exception_vector, int signo,
> > * over and over again.
> > */
> > kgdb_arch_update_addr(linux_regs, remcom_in_buffer);
> >- atomic_set(&kgdb_cpu_doing_single_step, -1);
> >- kgdb_single_step = 0;
> This is a subtle change, but I assume it is what you intended? All the CPUs will get released into the run state when exiting the kgdb exception handler.

You are talking about "- kgdb_single_step = 0." Right?
Do you think that there is any (negative) side effect of this change?
Since most of architectures, including x86, don't handle this variable
explicitly, I didn't expect that it was essential.

> >-
> >- /*
> >- * Received continue command, disable single step
> >- */
> >- if (kernel_active_single_step())
> >- kernel_disable_single_step();
> >
> I see why this is not needed above any more given that you have a function that cleans up the state on entry to the kgdb exception handler.

Yeah, the point here is that, on arm64, we need to set SS (Software step)
bit in SPSR as well as MDSCR before returning from the exception handler
in order to enable a hardware single step according to ARM ARM D2.12
("Software Step exceptions").
So it might be good enough just to call kernel_enable_single_step()
at 's' command unconditionally instead of "if (!kernel_active_single_step)".
(Please note that, as I mentioned in the commit message, SPSR.SS bit will
be cleared while MDSCR.SS bit is kept on after a hardware single step.)

But anyhow, I believe that the hunk above is redundant in my implementation.

> The rest of the patch is fine.

Thank you.

> I added the patch to kgdb-next after fixing up the context since it no longer applied to the mainline ( If there is further discussion on the point above, another patch can be added, but it I am assuming this is the way you desire it to work as there are some other architectures that use the same behaviour. I do not presently have any ARM64 hardware to validate this particular change.
> I also added to the patch a "Cc: linux-stable <stable@xxxxxxxxxxxxxxx>" so we can have this appear on some of the older kernels.

Since Will asked me to split this patch into a few, I need some reworks
to clarify which hunks in the patch are necessary for which version of kernel.

-Takahiro AKASHI

> Thanks,
> Jason.