linux-kernel - Re: [PATCH v5 17/22] powerpc/syscall: Do not check unsupported scv vector on PPC32

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Message-Id: <1612835741.qmlhg8iwmj.astroid@bobo.none>
Date:   Tue, 09 Feb 2021 12:00:03 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>, msuchanek@...e.de,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v5 17/22] powerpc/syscall: Do not check unsupported scv
 vector on PPC32

Excerpts from Christophe Leroy's message of February 9, 2021 1:10 am:
> Only PPC64 has scv. No need to check the 0x7ff0 trap on PPC32.
> For that, add a helper trap_is_unsupported_scv() similar to
> trap_is_scv().
> 
> And ignore the scv parameter in syscall_exit_prepare (Save 14 cycles
> 346 => 332 cycles)
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@...roup.eu>
> ---
> v5: Added a helper trap_is_unsupported_scv()
> ---
>  arch/powerpc/include/asm/ptrace.h | 5 +++++
>  arch/powerpc/kernel/entry_32.S    | 1 -
>  arch/powerpc/kernel/interrupt.c   | 7 +++++--
>  3 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/ptrace.h b/arch/powerpc/include/asm/ptrace.h
> index 58f9dc060a7b..2c842b11a924 100644
> --- a/arch/powerpc/include/asm/ptrace.h
> +++ b/arch/powerpc/include/asm/ptrace.h
> @@ -229,6 +229,11 @@ static inline bool trap_is_scv(struct pt_regs *regs)
>  	return (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && TRAP(regs) == 0x3000);
>  }
>  
> +static inline bool trap_is_unsupported_scv(struct pt_regs *regs)
> +{
> +	return (IS_ENABLED(CONFIG_PPC_BOOK3S_64) && TRAP(regs) == 0x7ff0);
> +}

This change is good.

> +
>  static inline bool trap_is_syscall(struct pt_regs *regs)
>  {
>  	return (trap_is_scv(regs) || TRAP(regs) == 0xc00);
> diff --git a/arch/powerpc/kernel/entry_32.S b/arch/powerpc/kernel/entry_32.S
> index cffe58e63356..7c824e8928d0 100644
> --- a/arch/powerpc/kernel/entry_32.S
> +++ b/arch/powerpc/kernel/entry_32.S
> @@ -344,7 +344,6 @@ transfer_to_syscall:
>  
>  ret_from_syscall:
>  	addi    r4,r1,STACK_FRAME_OVERHEAD
> -	li	r5,0
>  	bl	syscall_exit_prepare

For this one, I think it would be nice to do the "right" thing and make 
the function prototypes different on !64S. They could then declare a
local const bool scv = 0.

We could have syscall_exit_prepare and syscall_exit_prepare_maybe_scv
or something like that, 64s can use the latter one and the former can be
a wrapper that passes constant 0 for scv. Then we don't have different
prototypes for the same function, but you just have to make the 32-bit
version static inline and the 64-bit version exported to asm.

Thanks,
Nick

>  #if defined(CONFIG_4xx) || defined(CONFIG_BOOKE)
>  	/* If the process has its own DBCR0 value, load it up.  The internal
> diff --git a/arch/powerpc/kernel/interrupt.c b/arch/powerpc/kernel/interrupt.c
> index 205902052112..8fafca727b8b 100644
> --- a/arch/powerpc/kernel/interrupt.c
> +++ b/arch/powerpc/kernel/interrupt.c
> @@ -88,7 +88,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
>  	local_irq_enable();
>  
>  	if (unlikely(current_thread_info()->flags & _TIF_SYSCALL_DOTRACE)) {
> -		if (unlikely(regs->trap == 0x7ff0)) {
> +		if (unlikely(trap_is_unsupported_scv(regs))) {
>  			/* Unsupported scv vector */
>  			_exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
>  			return regs->gpr[3];
> @@ -111,7 +111,7 @@ notrace long system_call_exception(long r3, long r4, long r5,
>  		r8 = regs->gpr[8];
>  
>  	} else if (unlikely(r0 >= NR_syscalls)) {
> -		if (unlikely(regs->trap == 0x7ff0)) {
> +		if (unlikely(trap_is_unsupported_scv(regs))) {
>  			/* Unsupported scv vector */
>  			_exception(SIGILL, regs, ILL_ILLOPC, regs->nip);
>  			return regs->gpr[3];
> @@ -224,6 +224,9 @@ notrace unsigned long syscall_exit_prepare(unsigned long r3,
>  	unsigned long ti_flags;
>  	unsigned long ret = 0;
>  
> +	if (IS_ENABLED(CONFIG_PPC32))
> +		scv = 0;
> +
>  	CT_WARN_ON(ct_state() == CONTEXT_USER);
>  
>  #ifdef CONFIG_PPC64
> -- 
> 2.25.0
> 
>