Date:	Thu, 28 Jan 2010 18:10:57 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Jason Wessel <jason.wessel@...driver.com>
Cc:	linux-kernel@...r.kernel.org, kgdb-bugreport@...ts.sourceforge.net,
	mingo@...e.hu, "K.Prasad" <prasad@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH 1/4] x86,hw_breakpoint,kgdb: kgdb to use hw_breakpoint
	API

On Mon, Jan 25, 2010 at 10:26:37PM -0600, Jason Wessel wrote:
> @@ -466,7 +466,7 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
>  {
>  	int i, cpu, rc = NOTIFY_STOP;
>  	struct perf_event *bp;
> -	unsigned long dr7, dr6;
> +	unsigned long dr6;
>  	unsigned long *dr6_p;
>  
>  	/* The DR6 value is pointed by args->err */
> @@ -477,7 +477,6 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
>  	if ((dr6 & DR_TRAP_BITS) == 0)
>  		return NOTIFY_DONE;
>  
> -	get_debugreg(dr7, 7);
>  	/* Disable breakpoints during exception handling */
>  	set_debugreg(0UL, 7);
>  	/*
> @@ -525,7 +524,7 @@ static int __kprobes hw_breakpoint_handler(struct die_args *args)
>  	if (dr6 & (~DR_TRAP_BITS))
>  		rc = NOTIFY_DONE;
>  
> -	set_debugreg(dr7, 7);
> +	set_debugreg(__get_cpu_var(cpu_dr7), 7);
>  	put_cpu();



Good simplification, but that doesn't appear related to kgdb,
this should be done in a separate patch, for the perf/core tree.



>  static void kgdb_correct_hw_break(void)
>  {
> -	unsigned long dr7;
> -	int correctit = 0;
> -	int breakbit;
>  	int breakno;
>  
> -	get_debugreg(dr7, 7);
>  	for (breakno = 0; breakno < 4; breakno++) {
> -		breakbit = 2 << (breakno << 1);
> -		if (!(dr7 & breakbit) && breakinfo[breakno].enabled) {
> -			correctit = 1;
> -			dr7 |= breakbit;
> -			dr7 &= ~(0xf0000 << (breakno << 2));
> -			dr7 |= ((breakinfo[breakno].len << 2) |
> -				 breakinfo[breakno].type) <<
> -			       ((breakno << 2) + 16);
> -			set_debugreg(breakinfo[breakno].addr, breakno);
> -
> -		} else {
> -			if ((dr7 & breakbit) && !breakinfo[breakno].enabled) {
> -				correctit = 1;
> -				dr7 &= ~breakbit;
> -				dr7 &= ~(0xf0000 << (breakno << 2));
> -			}
> -		}
> +		struct perf_event *bp;
> +		struct arch_hw_breakpoint *info;
> +		int val;
> +		int cpu = raw_smp_processor_id();
> +		if (!breakinfo[breakno].enabled)
> +			continue;
> +		bp = *per_cpu_ptr(breakinfo[breakno].pev, cpu);
> +		info = counter_arch_bp(bp);
> +		if (bp->attr.disabled != 1)
> +			continue;
> +		bp->attr.bp_addr = breakinfo[breakno].addr;
> +		bp->attr.bp_len = breakinfo[breakno].len;
> +		bp->attr.bp_type = breakinfo[breakno].type;
> +		info->address = breakinfo[breakno].addr;
> +		info->len = breakinfo[breakno].len;
> +		info->type = breakinfo[breakno].type;
> +		val = arch_install_hw_breakpoint(bp);
> +		if (!val)
> +			bp->attr.disabled = 0;
>  	}
> -	if (correctit)
> -		set_debugreg(dr7, 7);
> +	hw_breakpoint_restore();



hw_breakpoint_restore() is used by KVM only, for now.
The cpu var cpu_debugreg[] contains values that
are only saved when KVM switches to a guest, then
this function is called when KVM switches back to the
host. I bet this is not the function you need.
In fact, I don't know what you intended to do there.



> @@ -278,27 +285,38 @@ kgdb_set_hw_break(unsigned long addr, int len, enum kgdb_bptype bptype)
>  
>  	switch (bptype) {
>  	case BP_HARDWARE_BREAKPOINT:
> -		type = 0;
> -		len  = 1;
> +		len = 1;
> +		breakinfo[i].type = X86_BREAKPOINT_EXECUTE;
>  		break;
>  	case BP_WRITE_WATCHPOINT:
> -		type = 1;
> +		breakinfo[i].type = X86_BREAKPOINT_WRITE;
>  		break;
>  	case BP_ACCESS_WATCHPOINT:
> -		type = 3;
> +		breakinfo[i].type = X86_BREAKPOINT_RW;
>  		break;



Would be nice to have bptype set to the generic flags
we have already in linux/hw_breakpoint.h:

enum {
	HW_BREAKPOINT_R = 1,
	HW_BREAKPOINT_W = 2,
	HW_BREAKPOINT_X = 4,
};


We have functions in x86 to do the conversion to
x86 values in arch/x86/kernel/hw_breakpoint.c

Nothing urgent though, as this patch is a regression fix,
this can be done later.



> +	switch (len) {
> +	case 1:
> +		breakinfo[i].len = X86_BREAKPOINT_LEN_1;
> +		break;
> +	case 2:
> +		breakinfo[i].len = X86_BREAKPOINT_LEN_2;
> +		break;
> +	case 4:
> +		breakinfo[i].len = X86_BREAKPOINT_LEN_4;
> +		break;
> +#ifdef CONFIG_X86_64
> +	case 8:
> +	breakinfo[i].len = X86_BREAKPOINT_LEN_8;
> +		break;
> +#endif



Same here, see arch_build_bp_info().
Actually, arch_validate_hwbkpt_settings() would do all
that for you. May require few changes though to adapt.

Actually, I don't understand why you encumber with this
breakinfo thing. Why not just keeping a per cpu array
of perf events? You have everything you need inside:
the generic breakpoint attributes in the attrs and
the arch info in the hw_perf_event struct inside.

Hence you would be able to use the x86 breakpoint API
we have already, arch_validate_hwbkpt_settings() does
everything for you. This is going to shrink your code
and then make it a stronger argument to pull request
as a not-that-one-liner regression fix late in the
process (which I must confess is my bad, firstly: I
did the regression and secondly: I should have
reviewed these fixes sooner).



> @@ -313,8 +331,21 @@ kgdb_set_hw_break(unsigned long addr, int len, enum kgdb_bptype bptype)
>   */
>  void kgdb_disable_hw_debug(struct pt_regs *regs)
>  {
> +	int i;
> +	int cpu = raw_smp_processor_id();
> +	struct perf_event *bp;
> +
>  	/* Disable hardware debugging while we are in kgdb: */
>  	set_debugreg(0UL, 7);
> +	for (i = 0; i < 4; i++) {
> +		if (!breakinfo[i].enabled)



See? Here you could use simply bp->attr.disabled instead
of playing with this breakinfo.


> @@ -378,7 +409,6 @@ int kgdb_arch_handle_exception(int e_vector, int signo, int err_code,
>  			       struct pt_regs *linux_regs)
>  {
>  	unsigned long addr;
> -	unsigned long dr6;
>  	char *ptr;
>  	int newPC;
>  
> @@ -448,10 +464,12 @@ single_step_cont(struct pt_regs *regs, struct die_args *args)
>  }
>  
>  static int was_in_debug_nmi[NR_CPUS];
> +static int recieved_hw_brk[NR_CPUS];
>  
>  static int __kgdb_notify(struct die_args *args, unsigned long cmd)
>  {
>  	struct pt_regs *regs = args->regs;
> +	unsigned long *dr6_p;
>  
>  	switch (cmd) {
>  	case DIE_NMI:
> @@ -485,16 +503,24 @@ static int __kgdb_notify(struct die_args *args, unsigned long cmd)
>  		break;
>  
>  	case DIE_DEBUG:
> -		if (atomic_read(&kgdb_cpu_doing_single_step) ==
> -		    raw_smp_processor_id()) {
> +		dr6_p = (unsigned long *)ERR_PTR(args->err);
> +		if (atomic_read(&kgdb_cpu_doing_single_step) != -1) {
> +			if (dr6_p && (*dr6_p & DR_STEP) == 0)
> +				return NOTIFY_DONE;
>  			if (user_mode(regs))
>  				return single_step_cont(regs, args);
>  			break;
> -		} else if (test_thread_flag(TIF_SINGLESTEP))
> +		} else if (test_thread_flag(TIF_SINGLESTEP)) {
>  			/* This means a user thread is single stepping
>  			 * a system call which should be ignored
>  			 */
>  			return NOTIFY_DONE;
> +		} else if (recieved_hw_brk[raw_smp_processor_id()] == 1) {
> +			recieved_hw_brk[raw_smp_processor_id()] = 0;
> +			return NOTIFY_STOP;
> +		} else if (dr6_p && (*dr6_p & DR_TRAP_BITS) == 0) {
> +			return NOTIFY_DONE;
> +		}



So this is the debug handler, right?


>  
> +static void kgdb_hw_bp(struct perf_event *bp, int nmi,
> +		       struct perf_sample_data *data,
> +		       struct pt_regs *regs)
> +{
> +	struct die_args args;
> +	int cpu = raw_smp_processor_id();
> +
> +	args.trapnr = 0;
> +	args.signr = 5;
> +	args.err = 0;
> +	args.regs = regs;
> +	args.str = "debug";
> +	recieved_hw_brk[cpu] = 0;
> +	if (__kgdb_notify(&args, DIE_DEBUG) == NOTIFY_STOP)
> +		recieved_hw_brk[cpu] = 1;
> +	else
> +		recieved_hw_brk[cpu] = 0;
> +}



And this looks like the perf event handler.

I'm confused by the logic here. We have the x86 breakpoint
handler which calls perf_bp_event which in turn will call
the above. The above calls __kgdb_notify(), but it will
also be called later as it is a debug notifier.



> +
>  /**
>   *	kgdb_arch_init - Perform any architecture specific initalization.
>   *
> @@ -539,7 +584,43 @@ static struct notifier_block kgdb_notifier = {
>   */
>  int kgdb_arch_init(void)
>  {
> -	return register_die_notifier(&kgdb_notifier);
> +	int i, cpu;
> +	int ret;
> +	struct perf_event_attr attr;
> +	struct perf_event **pevent;
> +
> +	ret = register_die_notifier(&kgdb_notifier);
> +	if (ret != 0)
> +		return ret;
> +	/*
> +	 * Pre-allocate the hw breakpoint structions in the non-atomic
> +	 * portion of kgdb because this operation requires mutexs to
> +	 * complete.
> +	 */
> +	attr.bp_addr = (unsigned long)kgdb_arch_init;
> +	attr.type = PERF_TYPE_BREAKPOINT;
> +	attr.bp_len = HW_BREAKPOINT_LEN_1;
> +	attr.bp_type = HW_BREAKPOINT_X;
> +	attr.disabled = 1;
> +	for (i = 0; i < 4; i++) {
> +		breakinfo[i].pev = register_wide_hw_breakpoint(&attr,
> +							       kgdb_hw_bp);



By calling this, you are reserving all the breakpoint slots.



> +		if (IS_ERR(breakinfo[i].pev)) {
> +			printk(KERN_ERR "kgdb: Could not allocate hw breakpoints\n");
> +			breakinfo[i].pev = NULL;
> +			kgdb_arch_exit();
> +			return -1;
> +		}
> +		for_each_online_cpu(cpu) {
> +			pevent = per_cpu_ptr(breakinfo[i].pev, cpu);
> +			pevent[0]->hw.sample_period = 1;
> +			if (pevent[0]->destroy != NULL) {
> +				pevent[0]->destroy = NULL;
> +				release_bp_slot(*pevent);



And then you release these, ok. We should find a proper
way for that later, but for now it should work.

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