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

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Date:	Sun, 28 Apr 2013 01:05:13 -0500
From:	Jacob Shin <jacob.shin@....com>
To:	Oleg Nesterov <oleg@...hat.com>
CC:	Ingo Molnar <mingo@...hat.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>, <x86@...nel.org>,
	Stephane Eranian <eranian@...gle.com>,
	Jiri Olsa <jolsa@...hat.com>, <linux-kernel@...r.kernel.org>,
	Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len >
 HW_BREAKPOINT_LEN_8

On Sat, Apr 27, 2013 at 06:10:28PM +0200, Oleg Nesterov wrote:
> On 04/27, Jacob Shin wrote:
> >
> > On Sat, Apr 27, 2013 at 05:05:10PM +0200, Oleg Nesterov wrote:
> > > ...
> > > > +	if (info->mask)
> > > > +		set_dr_addr_mask(0, i);
> > >
> > > I agree we should clear addr_mask anyway.
> > >
> > > But I am just curious, what if we do not? I mean what will the hardware
> > > do if this breakpoint was already disabled but the mask wasn't cleared?
> >
> > Oh, it is fine if we don't and we are not using breakpoints, however I
> > was trying to account for per-thread events sharing the same DR
> > register, in that case we don't want previous event's mask value still
> > in the MSR.
> 
> Aha, so CPU "remembers" the non-cleared mask and this can affect the
> next "enable". Thanks.
> 
> > > Suppose that the kernel was compiled without CONFIG_CPU_SUP_AMD.
> > > Then perf_event_open(attr => { .bp_len == 16 }) will succeed, but
> > > this breakpoint won't actually work as expected?
> >
> > Well, on non-AMD, even if we have CONFIG_CPU_SUP_AMD=y or not,
> > cpu_has_bpext (x86 CPUID check) will fail.
> >
> > On AMD, if we don't have CONFIG_CPU_SUP_AMD=y then we can't even boot.
> 
> Heh, I didn't know ;)
> 
> OK, I think the patch is fine then.
> 
> Except... cough, the last nit, I promise ;)
> 
> Currently this doesn't matter, the only caller of modify_user_hw_breakpoint()
> is ptrace, and it can't use bp_len != HW_BREAKPOINT_LEN_*.
> 
> But still, I think your patch needs a small fixlet,
> 
> 	- /* info->len == info->mask == 0 */
> 	+ info->mask = 0;
> 
> Or we can do this later.
> 
> And while this is purely cosmetic (feel free to ignore), perhaps we
> can join the bp_len checks and move cpu_has_bpext from _validate to
> _build, this looks a little bit cleaner imho. IOW,
> 
> 
> 	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;
> 		/* fallthrough */
> 	case HW_BREAKPOINT_LEN_1:
> 		info->len = X86_BREAKPOINT_LEN_1;
> 		break;
> 	case HW_BREAKPOINT_LEN_2:
> 		info->len = X86_BREAKPOINT_LEN_2;
> 		break;
> 	case HW_BREAKPOINT_LEN_4:
> 		info->len = X86_BREAKPOINT_LEN_4;
> 		break;
> #ifdef CONFIG_X86_64
> 	case HW_BREAKPOINT_LEN_8:
> 		info->len = X86_BREAKPOINT_LEN_8;
> 		break;
> #endif
> 	}
> 
> Then arch_validate_hwbkpt_settings() only needs
> 
> 		case X86_BREAKPOINT_LEN_1:
> 			align = 0;
> 	+	if (info->mask)
> 	+		align = mask;
> 
> change.

Okay I have made these changes and did final smoke testing .. I'll
send out the patch bomb in a bit.

Thanks again, and again for taking the time to look this over!

-Jacob

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