Messages in this thread | | | Date | Sat, 27 Apr 2013 18:10:28 +0200 | From | Oleg Nesterov <> | Subject | Re: [PATCH 1/3] perf/x86/amd: AMD support for bp_len > HW_BREAKPOINT_LEN_8 |
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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.
Oleg.
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