Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len > HW_BREAKPOINT_LEN_8

Jacob Shin
2013-04-28T06:06:18Z

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