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

["Nicholas Piggin" <npiggin@gmail.com>]

On Fri Mar 1, 2024 at 7:53 PM AEST, Thomas Huth wrote:
> 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 <npiggin@gmail.com>
> > ---
> >   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?

No, we switch to exception stack and don't support recursing interrupts
on stack at the moment, so this goes into the initial frame.

> Please add a comment here explaining this line. 
> Thanks!

Yes, good idea.

Thanks,
Nick

>
>
> >   	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