Subject: Re: [PATCH 1/2] [hw-breakpoint] Use generic hw-breakpoint interfaces for new PPC ptrace flags

From: David Gibson <[email-masked]>
Date: Mon, 28 Nov 2011 14:11:11 +1100

[snip]
On Wed, Oct 12, 2011 at 11:09:48PM +0530, K.Prasad wrote:
> > > +	if (bp) {
> > > +		attr = bp->attr;
> > > +		attr.bp_addr = (unsigned long)bp_info->addr & ~HW_BREAKPOINT_ALIGN;
> > > +		arch_bp_generic_fields(dabr &
> > > +					(DABR_DATA_WRITE | DABR_DATA_READ),
> > > +							&attr.bp_type);
> > > +		attr.bp_len = len;
> > 
> > If gdb is using the new breakpoint interface, surely it should just
> > use it, rather than doing this bit frobbing as in the old SET_DABR
> > call.
> > 
> 
> I understand that you wanted to avoid this duplication of effort in terms
> of encoding and decoding the breakpoint type from
> PPC_BREAKPOINT_TRIGGER_READ to DABR_DATA_READ to HW_BREAKPOINT_R.
> 
> However HW_BREAKPOINT_R is a generic definition used across
> architectures, DABR_DATA_READ is used in the !CONFIG_HAVE_HW_BREAKPOINT
> case while PPC_BREAKPOINT_TRIGGER_READ is used in
> CONFIG_PPC_ADV_DEBUG_REGS case.
> 
> While we could define PPC_BREAKPOINT_TRIGGER_READ and DABR_DATA_READ to
> the same value it may not result in any code savings (since the bit
> translation is done for !CONFIG_HAVE_HW_BREAKPOINT case anyway). So, I
> think it is best left the way it is.

That's not what I'm suggesting.  What I'm saying is that ig userspace
is using the new generic interface, then it should just set the
bp_type field, and it should not use the DABR_DATA_{READ,WRITE} bits.
The DABR_DATA bits should *only* be processed in the legacy interface,
never in the generic interface.

> I'm attaching the revised patch (after incorporating your comments).
> Kindly let me know your comments.
> 
> Thanks,
> K.Prasad
> ---------
> 
> [hw-breakpoint] Use generic hw-breakpoint interfaces for new PPC ptrace flags
> 
> PPC_PTRACE_GETHWDBGINFO, PPC_PTRACE_SETHWDEBUG and PPC_PTRACE_DELHWDEBUG are
> PowerPC specific ptrace flags that use the watchpoint register. While they are
> targeted primarily towards BookE users, user-space applications such as GDB
> have started using them for BookS too. This patch enables the use of generic
> hardware breakpoint interfaces for these new flags.
> 
> Apart from the usual benefits of using generic hw-breakpoint interfaces, these
> changes allow debuggers (such as GDB) to use a common set of ptrace flags for
> their watchpoint needs and allow more precise breakpoint specification (length
> of the variable can be specified).
> 
> Tested-by: Edjunior Barbosa Machado <[email-masked]>
> Signed-off-by: K.Prasad <[email-masked]>
> ---
>  Documentation/powerpc/ptrace.txt |   16 +++++++
>  arch/powerpc/kernel/ptrace.c     |   88 +++++++++++++++++++++++++++++++++++---
>  2 files changed, 98 insertions(+), 6 deletions(-)
> 
> diff --git a/Documentation/powerpc/ptrace.txt b/Documentation/powerpc/ptrace.txt
> index f4a5499..f2a7a39 100644
> --- a/Documentation/powerpc/ptrace.txt
> +++ b/Documentation/powerpc/ptrace.txt
> @@ -127,6 +127,22 @@ Some examples of using the structure to:
>    p.addr2           = (uint64_t) end_range;
>    p.condition_value = 0;
>  
> +- set a watchpoint in server processors (BookS)
> +
> +  p.version         = 1;
> +  p.trigger_type    = PPC_BREAKPOINT_TRIGGER_RW;
> +  p.addr_mode       = PPC_BREAKPOINT_MODE_RANGE_INCLUSIVE;
> +  or
> +  p.addr_mode       = PPC_BREAKPOINT_MODE_EXACT;
> +
> +  p.condition_mode  = PPC_BREAKPOINT_CONDITION_NONE;
> +  p.addr            = (uint64_t) begin_range;

You should probably document the alignment constraint on the address
here, too.

> +  /* For PPC_BREAKPOINT_MODE_RANGE_INCLUSIVE addr2 needs to be specified, where
> +   * addr2 - addr <= 8 Bytes.
> +   */
> +  p.addr2           = (uint64_t) end_range;
> +  p.condition_value = 0;
> +
>  3. PTRACE_DELHWDEBUG
>  
>  Takes an integer which identifies an existing breakpoint or watchpoint
> diff --git a/arch/powerpc/kernel/ptrace.c b/arch/powerpc/kernel/ptrace.c
> index 05b7dd2..be5dc57 100644
> --- a/arch/powerpc/kernel/ptrace.c
> +++ b/arch/powerpc/kernel/ptrace.c
> @@ -1339,6 +1339,12 @@ static int set_dac_range(struct task_struct *child,
>  static long ppc_set_hwdebug(struct task_struct *child,
>  		     struct ppc_hw_breakpoint *bp_info)
>  {
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	int ret, len = 0;
> +	struct thread_struct *thread = &(child->thread);
> +	struct perf_event *bp;
> +	struct perf_event_attr attr;
> +#endif /* CONFIG_HAVE_HW_BREAKPOINT */
>  #ifndef CONFIG_PPC_ADV_DEBUG_REGS
>  	unsigned long dabr;
>  #endif
> @@ -1382,13 +1388,9 @@ static long ppc_set_hwdebug(struct task_struct *child,
>  	 */
>  	if ((bp_info->trigger_type & PPC_BREAKPOINT_TRIGGER_RW) == 0 ||
>  	    (bp_info->trigger_type & ~PPC_BREAKPOINT_TRIGGER_RW) != 0 ||
> -	    bp_info->addr_mode != PPC_BREAKPOINT_MODE_EXACT ||
>  	    bp_info->condition_mode != PPC_BREAKPOINT_CONDITION_NONE)
>  		return -EINVAL;
>  
> -	if (child->thread.dabr)
> -		return -ENOSPC;
> -
>  	if ((unsigned long)bp_info->addr >= TASK_SIZE)
>  		return -EIO;
>  
> @@ -1398,15 +1400,75 @@ static long ppc_set_hwdebug(struct task_struct *child,
>  		dabr |= DABR_DATA_READ;
>  	if (bp_info->trigger_type & PPC_BREAKPOINT_TRIGGER_WRITE)
>  		dabr |= DABR_DATA_WRITE;
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	if (ptrace_get_breakpoints(child) < 0)
> +		return -ESRCH;
>  
> -	child->thread.dabr = dabr;
> +	/*
> +	 * Check if the request is for 'range' breakpoints. We can
> +	 * support it if range < 8 bytes.
> +	 */
> +	if (bp_info->addr_mode == PPC_BREAKPOINT_MODE_RANGE_INCLUSIVE) {
> +		len = bp_info->addr2 - bp_info->addr;
> +	} else if (bp_info->addr_mode != PPC_BREAKPOINT_MODE_EXACT) {
> +			ptrace_put_breakpoints(child);
> +			return -EINVAL;

You are overindented here.

> +	}
> +	bp = thread->ptrace_bps[0];
> +	if (bp) {
> +		attr = bp->attr;
> +		attr.bp_addr = (unsigned long)bp_info->addr & ~HW_BREAKPOINT_ALIGN;
> +		arch_bp_generic_fields(dabr &
> +					(DABR_DATA_WRITE | DABR_DATA_READ),
> +							&attr.bp_type);

You still have this code which has no business in the generic
interface path.

> +		attr.bp_len = len;
> +		ret =  modify_user_hw_breakpoint(bp, &attr);
> +		if (ret) {
> +			ptrace_put_breakpoints(child);
> +			return ret;
> +		}

If a bp already exists, you're modifying it.  I thought the semantics
of the new interface meant that you shoul return ENOSPC in this case,
and a DEL would be necessary before adding another breakpoint.

> +		thread->ptrace_bps[0] = bp;
> +		ptrace_put_breakpoints(child);
> +		thread->dabr = dabr;
> +		return 0;
> +	}
> +
> +	/* Create a new breakpoint request if one doesn't exist already */
> +	hw_breakpoint_init(&attr);
> +	attr.bp_addr = (unsigned long)bp_info->addr & ~HW_BREAKPOINT_ALIGN;
> +	attr.bp_len = len;
> +	arch_bp_generic_fields(dabr & (DABR_DATA_WRITE | DABR_DATA_READ),
> +								&attr.bp_type);
> +
> +	thread->ptrace_bps[0] = bp = register_user_hw_breakpoint(&attr,
> +					       ptrace_triggered, NULL, child);
> +	if (IS_ERR(bp)) {
> +		thread->ptrace_bps[0] = NULL;
> +		ptrace_put_breakpoints(child);
> +		return PTR_ERR(bp);
> +	}
>  
> +	ptrace_put_breakpoints(child);
> +	return 1;
> +#endif /* CONFIG_HAVE_HW_BREAKPOINT */
> +
> +	if (bp_info->addr_mode != PPC_BREAKPOINT_MODE_EXACT)
> +		return -EINVAL;
> +
> +	if (child->thread.dabr)
> +		return -ENOSPC;
> +
> +	child->thread.dabr = dabr;
>  	return 1;
>  #endif /* !CONFIG_PPC_ADV_DEBUG_DVCS */
>  }
>  
>  static long ppc_del_hwdebug(struct task_struct *child, long addr, long data)
>  {
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	struct thread_struct *thread = &(child->thread);
> +	struct perf_event *bp;
> +#endif /* CONFIG_HAVE_HW_BREAKPOINT */
>  #ifdef CONFIG_PPC_ADV_DEBUG_REGS
>  	int rc;
>  
> @@ -1426,10 +1488,24 @@ static long ppc_del_hwdebug(struct task_struct *child, long addr, long data)
>  #else
>  	if (data != 1)
>  		return -EINVAL;
> +
> +#ifdef CONFIG_HAVE_HW_BREAKPOINT
> +	if (ptrace_get_breakpoints(child) < 0)
> +		return -ESRCH;
> +
> +	bp = thread->ptrace_bps[0];
> +	if (bp) {
> +		unregister_hw_breakpoint(bp);
> +		thread->ptrace_bps[0] = NULL;
> +	}
> +	ptrace_put_breakpoints(child);
> +	return 0;

Shouldn't DEL return an error if there is no existing bp.

> +#else /* CONFIG_HAVE_HW_BREAKPOINT */
>  	if (child->thread.dabr == 0)
>  		return -ENOENT;
>  
>  	child->thread.dabr = 0;
> +#endif /* CONFIG_HAVE_HW_BREAKPOINT */
>  
>  	return 0;
>  #endif
> @@ -1560,7 +1636,7 @@ long arch_ptrace(struct task_struct *child, long request,
>  		dbginfo.data_bp_alignment = 4;
>  #endif
>  		dbginfo.sizeof_condition = 0;
> -		dbginfo.features = 0;
> +		dbginfo.features = PPC_DEBUG_FEATURE_DATA_BP_RANGE;
>  #endif /* CONFIG_PPC_ADV_DEBUG_REGS */
>  
>  		if (!access_ok(VERIFY_WRITE, datavp,

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson