Re: [RFC 0/5] Introduce NMI aware serial drivers

[Sumit Garg <sumit.garg@linaro.org>]

On Tue, 21 Jul 2020 at 17:40, Sumit Garg <sumit.garg@linaro.org> wrote:
>
> Make it possible for UARTs to trigger magic sysrq from an NMI. With the
> advent of pseudo NMIs on arm64 it became quite generic to request serial
> device interrupt as an NMI rather than IRQ. And having NMI driven serial
> RX will allow us to trigger magic sysrq as an NMI and hence drop into
> kernel debugger in NMI context.
>
> The major use-case is to add NMI debugging capabilities to the kernel
> in order to debug scenarios such as:
> - Primary CPU is stuck in deadlock with interrupts disabled and hence
>   doesn't honor serial device interrupt. So having magic sysrq triggered
>   as an NMI is helpful for debugging.
> - Always enabled NMI based magic sysrq irrespective of whether the serial
>   TTY port is active or not.
>
> Currently there is an existing kgdb NMI serial driver which provides
> partial implementation in upstream to have a separate ttyNMI0 port but
> that remained in silos with the serial core/drivers which made it a bit
> odd to enable using serial device interrupt and hence remained unused. It
> seems to be clearly intended to avoid almost all custom NMI changes to
> the UART driver.
>
> But this patch-set allows the serial core/drivers to be NMI aware which
> in turn provides NMI debugging capabilities via magic sysrq and hence
> there is no specific reason to keep this special driver. So remove it
> instead.
>
> Approach:
> ---------
>
> The overall idea is to intercept serial RX characters in NMI context, if
> those are specific to magic sysrq then allow corresponding handler to run
> in NMI context. Otherwise, defer all other RX and TX operations onto IRQ
> work queue in order to run those in normal interrupt context.
>
> This approach is demonstrated using amba-pl011 driver.
>
> Patch-wise description:
> -----------------------
>
> Patch #1 prepares magic sysrq handler to be NMI aware.
> Patch #2 adds NMI framework to serial core.
> Patch #3 and #4 demonstrates NMI aware uart port using amba-pl011 driver.
> Patch #5 removes kgdb NMI serial driver.
>
> Goal of this RFC:
> -----------------
>
> My main reason for sharing this as an RFC is to help decide whether or
> not to continue with this approach. The next step for me would to port
> the work to a system with an 8250 UART.
>

A gentle reminder to seek feedback on this series.

-Sumit

> Usage:
> ------
>
> This RFC has been developed on top of 5.8-rc3 and if anyone is interested
> to give this a try on QEMU, just enable following config options
> additional to arm64 defconfig:
>
> CONFIG_KGDB=y
> CONFIG_KGDB_KDB=y
> CONFIG_ARM64_PSEUDO_NMI=y
>
> Qemu command line to test:
>
> $ qemu-system-aarch64 -nographic -machine virt,gic-version=3 -cpu cortex-a57 \
>   -smp 2 -kernel arch/arm64/boot/Image -append 'console=ttyAMA0,38400 \
>   keep_bootcon root=/dev/vda2 irqchip.gicv3_pseudo_nmi=1 kgdboc=ttyAMA0' \
>   -initrd rootfs-arm64.cpio.gz
>
> NMI entry into kgdb via sysrq:
> - Ctrl a + b + g
>
> Reference:
> ----------
>
> For more details about NMI/FIQ debugger, refer to this blog post [1].
>
> [1] https://www.linaro.org/blog/debugging-arm-kernels-using-nmifiq/
>
> I do look forward to your comments and feedback.
>
> Sumit Garg (5):
>   tty/sysrq: Make sysrq handler NMI aware
>   serial: core: Add framework to allow NMI aware serial drivers
>   serial: amba-pl011: Re-order APIs definition
>   serial: amba-pl011: Enable NMI aware uart port
>   serial: Remove KGDB NMI serial driver
>
>  drivers/tty/serial/Kconfig       |  19 --
>  drivers/tty/serial/Makefile      |   1 -
>  drivers/tty/serial/amba-pl011.c  | 232 +++++++++++++++++-------
>  drivers/tty/serial/kgdb_nmi.c    | 383 ---------------------------------------
>  drivers/tty/serial/kgdboc.c      |   8 -
>  drivers/tty/serial/serial_core.c | 120 +++++++++++-
>  drivers/tty/sysrq.c              |  33 +++-
>  include/linux/kgdb.h             |  10 -
>  include/linux/serial_core.h      |  67 +++++++
>  include/linux/sysrq.h            |   1 +
>  kernel/debug/debug_core.c        |   1 +
>  11 files changed, 386 insertions(+), 489 deletions(-)
>  delete mode 100644 drivers/tty/serial/kgdb_nmi.c
>
> --
> 2.7.4
>