Re: [PATCH] lib/sstep: Fix incorrect return from analyze_instr()

[Michael Ellerman <mpe@ellerman.id.au>]

Ananth N Mavinakayanahalli <ananth@linux.ibm.com> writes:
> We currently just percolate the return value from analyze_instr()
> to the caller of emulate_step(), especially if it is a -1.
>
> For one particular case (opcode = 4) for instructions that
> aren't currently emulated, we are returning 'should not be
> single-stepped' while we should have returned 0 which says
> 'did not emulate, may have to single-step'.
>
> Signed-off-by: Ananth N Mavinakayanahalli <ananth@linux.ibm.com>
> Tested-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
> ---
>  arch/powerpc/lib/sstep.c |   49 +++++++++++++++++++++++++---------------------
>  1 file changed, 27 insertions(+), 22 deletions(-)
>
> diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
> index 5a425a4a1d88..a3a0373843cd 100644
> --- a/arch/powerpc/lib/sstep.c
> +++ b/arch/powerpc/lib/sstep.c
> @@ -1445,34 +1445,39 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
>  
>  #ifdef __powerpc64__
>  	case 4:
> -		if (!cpu_has_feature(CPU_FTR_ARCH_300))
> -			return -1;
> -
> -		switch (word & 0x3f) {
> -		case 48:	/* maddhd */
> -			asm volatile(PPC_MADDHD(%0, %1, %2, %3) :
> -				     "=r" (op->val) : "r" (regs->gpr[ra]),
> -				     "r" (regs->gpr[rb]), "r" (regs->gpr[rc]));
> -			goto compute_done;
> +		/*
> +		 * There are very many instructions with this primary opcode
> +		 * introduced in the ISA as early as v2.03. However, the ones
> +		 * we currently emulate were all introduced with ISA 3.0
> +		 */
> +		if (cpu_has_feature(CPU_FTR_ARCH_300)) {
> +			switch (word & 0x3f) {
> +			case 48:	/* maddhd */
> +				asm volatile(PPC_MADDHD(%0, %1, %2, %3) :
> +					     "=r" (op->val) : "r" (regs->gpr[ra]),
> +					     "r" (regs->gpr[rb]), "r" (regs->gpr[rc]));
> +				goto compute_done;

Indenting everything makes this patch harder to read, and I think makes
the resulting code harder to read too. We already have two levels of
switch here, and we're inside a ~1700 line function, so keeping things
simple is important I think.

Doesn't this achieve the same result?

diff --git a/arch/powerpc/lib/sstep.c b/arch/powerpc/lib/sstep.c
index bf7a7d62ae8b..d631baaf1da2 100644
--- a/arch/powerpc/lib/sstep.c
+++ b/arch/powerpc/lib/sstep.c
@@ -1443,8 +1443,10 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 
 #ifdef __powerpc64__
 	case 4:
-		if (!cpu_has_feature(CPU_FTR_ARCH_300))
-			return -1;
+		if (!cpu_has_feature(CPU_FTR_ARCH_300)) {
+			op->type = UNKNOWN;
+			return 0;
+		}
 
 		switch (word & 0x3f) {
 		case 48:	/* maddhd */
@@ -1470,7 +1472,8 @@ int analyse_instr(struct instruction_op *op, const struct pt_regs *regs,
 		 * There are other instructions from ISA 3.0 with the same
 		 * primary opcode which do not have emulation support yet.
 		 */
-		return -1;
+		op->type = UNKNOWN;
+		return 0;
 #endif
 
 	case 7:		/* mulli */


cheers