Subject: Re: [PATCH V4 08/10] powerpc, perf: Enable SW filtering in branch stack sampling framework

From: Michael Ellerman <[email-masked]>
Date: Tue, 24 Dec 2013 14:29:50 +1100

On Fri, 2013-12-20 at 16:31 +0530, Anshuman Khandual wrote:
> On 12/09/2013 11:51 AM, Michael Ellerman wrote:
> > On Wed, 2013-04-12 at 10:32:40 UTC, Anshuman Khandual wrote:
> >> +
> >> +	if (bhrb_sw_filter & PERF_SAMPLE_BRANCH_IND_CALL) {
> >> +		/* XL-form instruction */
> >> +		if (instr_is_branch_xlform(*addr)) {
> >> +
> >> +			/* LR should be set */
> >> +			if (is_branch_link_set(*addr)) {
> >> +				/*
> >> +			 	 * Conditional and unconditional
> >> +			 	 * branch to CTR.
> >> +			 	 */
> >> +				if (is_xlform_ctr(*addr))
> >> +					result = true;
> >> +
> >> +				/*
> >> +			 	 * Conditional and unconditional
> >> +			 	 * branch to LR.
> >> +			 	 */
> >> +				if (is_xlform_lr(*addr))
> >> +					result = true;
> >> +
> >> +				/*
> >> +			 	 * Conditional and unconditional
> >> +			 	 * branch to TAR.
> >> +			 	 */
> >> +				if (is_xlform_tar(*addr))
> >> +					result = true;
> > 
> > What other kind of XL-Form branch is there?
> 
> I am not sure. Do you know of any ?

That was my point. There are no other types, so you can just do:

	if (bhrb_sw_filter & PERF_SAMPLE_BRANCH_IND_CALL)
		if (instr_is_branch_xlform(*addr) && is_branch_link_set(*addr))
			return true;

> >> +	if (bhrb_sw_filter & PERF_SAMPLE_BRANCH_COND) {
> >> +
> >> +		/* I-form instruction - excluded */
> >> +		if (instr_is_branch_iform(*addr))
> >> +			goto out;
> >> +
> >> +		/* B-form or XL-form instruction */
> >> +		if (instr_is_branch_bform(*addr) || instr_is_branch_xlform(*addr))  {
> >> +
> >> +			/* Not branch always  */
> >> +			if (!is_bo_always(*addr)) {
> >> +
> >> +				/* Conditional branch to CTR register */
> >> +				if (is_bo_ctr(*addr))
> >> +					goto out;
> > 
> > We might have discussed this but why not?
> 
> Did not get that, discuss what ?

Why are we saying a conditional branch to the CTR is not a conditional branch?

It is conditional, so I think it should be included.

> >> +
> >> +				/* CR[BI] conditional branch with static hint */
> > 
> > A conditional branch with a static hint is still a conditional branch?
> 
> No its not. 

Yes it is?

In fact they could be very interesting branches. Because the compiler or
programmer has statically hinted them, if the hint is wrong they may be a major
source of branch midpredicts.

> >> +				if (is_bo_crbi_off(*addr) || is_bo_crbi_on(*addr)) {
> >> +					if (is_bo_crbi_hint(*addr))
> >> +						goto out;
> >> +				}
> >> +
> >> +				result = true;
> >> +			}
> >> +		}
> >> +	}
> >> +out:
> >> +	return result;
> >> +}
 
> >> +	} else {
> >> +		/*
> >> +		 * Userspace address needs to be
> >> +		 * copied first before analysis.
> >> +		 */
> >> +		pagefault_disable();
> >> +		ret =  __get_user_inatomic(instr, (unsigned int __user *)addr);
> > 
> > I suspect you borrowed this incantation from the callchain code. Unlike that
> > code you don't fallback to reading the page tables directly.
> > 
> > I'd rather see the accessor in the callchain code made generic and have you
> > call it here.
> 
> You have mentioned to take care of this issue yourself.

Yes I will.

cheers