On Tue, 21 Jul 2020 at 17:40, Sumit Garg <sumit.garg@xxxxxxxxxx> 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 >
