Subject: Re: [kvm-unit-tests PATCH 04/32] powerpc: interrupt stack backtracing

From: Thomas Huth <[email-masked]>
Date: Fri, 1 Mar 2024 10:53:27 +0100

On 26/02/2024 11.11, Nicholas Piggin wrote:
> Add support for backtracing across interrupt stacks, and
> add interrupt frame backtrace for unhandled interrupts.
> 
> Signed-off-by: Nicholas Piggin <[email-masked]>
> ---
>   lib/powerpc/processor.c |  4 ++-
>   lib/ppc64/asm/stack.h   |  3 +++
>   lib/ppc64/stack.c       | 55 +++++++++++++++++++++++++++++++++++++++++
>   powerpc/Makefile.ppc64  |  1 +
>   powerpc/cstart64.S      |  7 ++++--
>   5 files changed, 67 insertions(+), 3 deletions(-)
>   create mode 100644 lib/ppc64/stack.c
> 
> diff --git a/lib/powerpc/processor.c b/lib/powerpc/processor.c
> index ad0d95666..114584024 100644
> --- a/lib/powerpc/processor.c
> +++ b/lib/powerpc/processor.c
> @@ -51,7 +51,9 @@ void do_handle_exception(struct pt_regs *regs)
>   		return;
>   	}
>   
> -	printf("unhandled cpu exception %#lx at NIA:0x%016lx MSR:0x%016lx\n", regs->trap, regs->nip, regs->msr);
> +	printf("Unhandled cpu exception %#lx at NIA:0x%016lx MSR:0x%016lx\n",
> +			regs->trap, regs->nip, regs->msr);
> +	dump_frame_stack((void *)regs->nip, (void *)regs->gpr[1]);
>   	abort();
>   }
>   
> diff --git a/lib/ppc64/asm/stack.h b/lib/ppc64/asm/stack.h
> index 9734bbb8f..94fd1021c 100644
> --- a/lib/ppc64/asm/stack.h
> +++ b/lib/ppc64/asm/stack.h
> @@ -5,4 +5,7 @@
>   #error Do not directly include <asm/stack.h>. Just use <stack.h>.
>   #endif
>   
> +#define HAVE_ARCH_BACKTRACE
> +#define HAVE_ARCH_BACKTRACE_FRAME
> +
>   #endif
> diff --git a/lib/ppc64/stack.c b/lib/ppc64/stack.c
> new file mode 100644
> index 000000000..fcb7fa860
> --- /dev/null
> +++ b/lib/ppc64/stack.c
> @@ -0,0 +1,55 @@
> +#include <libcflat.h>
> +#include <asm/ptrace.h>
> +#include <stack.h>
> +
> +extern char exception_stack_marker[];
> +
> +int backtrace_frame(const void *frame, const void **return_addrs, int max_depth)
> +{
> +	static int walking;
> +	int depth = 0;
> +	const unsigned long *bp = (unsigned long *)frame;
> +	void *return_addr;
> +
> +	asm volatile("" ::: "lr"); /* Force it to save LR */
> +
> +	if (walking) {
> +		printf("RECURSIVE STACK WALK!!!\n");
> +		return 0;
> +	}
> +	walking = 1;
> +
> +	bp = (unsigned long *)bp[0];
> +	return_addr = (void *)bp[2];
> +
> +	for (depth = 0; bp && depth < max_depth; depth++) {
> +		return_addrs[depth] = return_addr;
> +		if (return_addrs[depth] == 0)
> +			break;
> +		if (return_addrs[depth] == exception_stack_marker) {
> +			struct pt_regs *regs;
> +
> +			regs = (void *)bp + STACK_FRAME_OVERHEAD;
> +			bp = (unsigned long *)bp[0];
> +			/* Represent interrupt frame with vector number */
> +			return_addr = (void *)regs->trap;
> +			if (depth + 1 < max_depth) {
> +				depth++;
> +				return_addrs[depth] = return_addr;
> +				return_addr = (void *)regs->nip;
> +			}
> +		} else {
> +			bp = (unsigned long *)bp[0];
> +			return_addr = (void *)bp[2];
> +		}
> +	}
> +
> +	walking = 0;
> +	return depth;
> +}
> +
> +int backtrace(const void **return_addrs, int max_depth)
> +{
> +	return backtrace_frame(__builtin_frame_address(0), return_addrs,
> +			       max_depth);
> +}
> diff --git a/powerpc/Makefile.ppc64 b/powerpc/Makefile.ppc64
> index b0ed2b104..eb682c226 100644
> --- a/powerpc/Makefile.ppc64
> +++ b/powerpc/Makefile.ppc64
> @@ -17,6 +17,7 @@ cstart.o = $(TEST_DIR)/cstart64.o
>   reloc.o  = $(TEST_DIR)/reloc64.o
>   
>   OBJDIRS += lib/ppc64
> +cflatobjs += lib/ppc64/stack.o
>   
>   # ppc64 specific tests
>   tests = $(TEST_DIR)/spapr_vpa.elf
> diff --git a/powerpc/cstart64.S b/powerpc/cstart64.S
> index 14ab0c6c8..278af84a6 100644
> --- a/powerpc/cstart64.S
> +++ b/powerpc/cstart64.S
> @@ -188,6 +188,7 @@ call_handler:
>   	.endr
>   	mfsprg1	r0
>   	std	r0,GPR1(r1)
> +	std	r0,0(r1)
>   
>   	/* lr, xer, ccr */
>   
> @@ -206,12 +207,12 @@ call_handler:
>   	subi	r31, r31, 0b - start_text
>   	ld	r2, (p_toc_text - start_text)(r31)
>   
> -	/* FIXME: build stack frame */
> -
>   	/* call generic handler */
>   
>   	addi	r3,r1,STACK_FRAME_OVERHEAD
>   	bl	do_handle_exception
> +	.global exception_stack_marker
> +exception_stack_marker:
>   
>   	/* restore context */
>   
> @@ -321,6 +322,7 @@ handler_trampoline:
>   	/* nip and msr */
>   	mfsrr0	r0
>   	std	r0, _NIP(r1)
> +	std	r0, INT_FRAME_SIZE+16(r1)

So if I got that right, INT_FRAME_SIZE+16 points to the stack frame of the 
function that was running before the interrupt handler? Is it such a good 
idea to change that here? Please add a comment here explaining this line. 
Thanks!

>   	mfsrr1	r0
>   	std	r0, _MSR(r1)
> @@ -337,6 +339,7 @@ handler_htrampoline:
>   	/* nip and msr */
>   	mfspr	r0, SPR_HSRR0
>   	std	r0, _NIP(r1)
> +	std	r0, INT_FRAME_SIZE+16(r1)

dito.

>   	mfspr	r0, SPR_HSRR1
>   	std	r0, _MSR(r1)

  Thomas