linux-kernel - Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len > HW_BREAKPOINT_LEN

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Date:	Tue, 10 Dec 2013 17:30:53 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	suravee.suthikulpanit@....com, mingo@...nel.org, mingo@...hat.com,
	jacob.w.shin@...il.com, a.p.zijlstra@...llo.nl,
	acme@...stprotocols.net, hpa@...or.com, tgl@...ain.invalid,
	linux-kernel@...r.kernel.org, sherry.hurwitz@....com
Subject: Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len >
 HW_BREAKPOINT_LEN_8

On Tue, Dec 10, 2013 at 05:19:45PM +0100, Oleg Nesterov wrote:
> On 12/10, Frederic Weisbecker wrote:
> >
> > On Wed, Oct 02, 2013 at 11:11:06AM -0500, suravee.suthikulpanit@....com wrote:
> > > @@ -279,7 +260,16 @@ static int arch_build_bp_info(struct perf_event *bp)
> > >  	}
> > >
> > >  	/* Len */
> > > +	info->mask = 0;
> > > +
> > >  	switch (bp->attr.bp_len) {
> > > +	default:
> > > +		if (!is_power_of_2(bp->attr.bp_len))
> > > +			return -EINVAL;
> > > +		if (!cpu_has_bpext)
> > > +			return -EOPNOTSUPP;
> > > +		info->mask = bp->attr.bp_len - 1;
> > > +		/* fall through */
> >
> > So, that's perhaps just personal preference but having the default on
> > top of the switch makes things very confusing. Can't we put the above
> > in the end of the switch instead?
> 
> Then "fall through" won't work ;)

Indeed, now may be that's just me but it's very hard to parse :)

There are other ways to perform the above, it's no big deal if
we duplicate one line of code.

> 
> > > @@ -314,11 +303,11 @@ int arch_validate_hwbkpt_settings(struct perf_event *bp)
> > >  	if (ret)
> > >  		return ret;
> > >
> > > -	ret = -EINVAL;
> > > -
> > >  	switch (info->len) {
> > >  	case X86_BREAKPOINT_LEN_1:
> > >  		align = 0;
> > > +		if (info->mask)
> > > +			align = info->mask;
> >
> > Confused, I thought mask is set only when length is above 8?
> 
> Yes. But we need the info->len for hw anyway. if mask != 0 then
> len == X86_BREAKPOINT_LEN_1 and it is still used by encode_dr7().
> Note that it is not the length in bytes, it is the magic x86 code.

Good point, and that matches the above fallthrough.

Thanks.

> 
> ->bp_len is the length.
> 
> Oleg.
> 
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