Re: [PATCH v2 1/3] perf: Add perf_event_attr::bp_priv

[Tiezhu Yang <yangtiezhu@loongson.cn>]

On 07/08/2024 03:36 PM, Peter Zijlstra wrote:
> On Sat, Jul 06, 2024 at 01:31:03PM +0800, Tiezhu Yang wrote:
>>
>>
>> On 07/05/2024 06:34 PM, Will Deacon wrote:
>>> On Fri, Jun 21, 2024 at 03:39:08PM +0800, Tiezhu Yang wrote:
>>>> Add a member "bp_priv" at the end of the uapi struct perf_event_attr
>>>> to make a bridge between ptrace and hardware breakpoint.
>>>>
>>>> This is preparation for later patch on some archs such as ARM, ARM64
>>>> and LoongArch which have privilege level of breakpoint.
>>>>
>>>> Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
>>>> ---
>>>>  include/uapi/linux/perf_event.h | 3 +++
>>>>  kernel/events/hw_breakpoint.c   | 1 +
>>>>  2 files changed, 4 insertions(+)
>>>>
>>>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>>>> index 3a64499b0f5d..f9f917e854e6 100644
>>>> --- a/include/uapi/linux/perf_event.h
>>>> +++ b/include/uapi/linux/perf_event.h
>>>> @@ -379,6 +379,7 @@ enum perf_event_read_format {
>>>>  #define PERF_ATTR_SIZE_VER6	120	/* add: aux_sample_size */
>>>>  #define PERF_ATTR_SIZE_VER7	128	/* add: sig_data */
>>>>  #define PERF_ATTR_SIZE_VER8	136	/* add: config3 */
>>>> +#define PERF_ATTR_SIZE_VER9	144	/* add: bp_priv */
>>>>
>>>>  /*
>>>>   * Hardware event_id to monitor via a performance monitoring event:
>>>> @@ -522,6 +523,8 @@ struct perf_event_attr {
>>>>  	__u64	sig_data;
>>>>
>>>>  	__u64	config3; /* extension of config2 */
>>>> +
>>>> +	__u8	bp_priv; /* privilege level of breakpoint */
>>>>  };
>>>
>>> Why are we extending the user ABI for this? Perf events already have the
>>> privilege encoded (indirectly) by the exclude_{user,kernel,hv} fields in
>>> 'struct perf_event_attr'.
>>
>> IMO, add bp_priv is to keep consistent with the other fields
>> bp_type, bp_addr and bp_len, the meaning of bp_priv field is
>> explicit and different with exclude_{user,kernel,hv} fields.
>
> In case it wasn't obvious, this structure has __u64 granularity. You
> don't just add a __u8 to the end. Also, since you mention consistency,
> you might have noticed those other bp_ fields are in a union on
> config[12], so why can't this live in a union on config3 ?

Looks good, I will do it in v3, like this:
(no need to define PERF_ATTR_SIZE_VER9	144)

diff --git a/include/uapi/linux/perf_event.h 
b/include/uapi/linux/perf_event.h
index 3a64499b0f5d..abe8da7a1f60 100644
--- a/include/uapi/linux/perf_event.h
+++ b/include/uapi/linux/perf_event.h
@@ -521,7 +521,10 @@ struct perf_event_attr {
          */
         __u64   sig_data;

-       __u64   config3; /* extension of config2 */
+       union {
+               __u8    bp_priv; /* privilege level of breakpoint */
+               __u64   config3; /* extension of config2 */
+       };
  };

>> Additionally, there is only 1 bit for exclude_{user,kernel,hv},
>> but bp_priv field has at least 2 bit according to the explanation
>> of Arm Reference Manual. At last, the initial aim is to remove
>> the check condition to assign the value of hw->ctrl.privilege.
>>
>> https://developer.arm.com/documentation/ddi0487/latest/
>>
>> 1. D23: AArch64 System Register Descriptions (Page 8562)
>>    D23.3.11 DBGWCR<n>_EL1, Debug Watchpoint Control Registers, n = 0 - 63
>>    PAC, bits [2:1]
>>    Privilege of access control. Determines the Exception level or levels at
>> which a Watchpoint debug
>>    event for watchpoint n is generated.
>>
>> 2. G8: AArch32 System Register Descriptions (Page 12334)
>>    G8.3.26 DBGWCR<n>, Debug Watchpoint Control Registers, n = 0 - 15
>>    PAC, bits [2:1]
>>    Privilege of access control. Determines the Exception level or levels at
>> which a Watchpoint debug
>>    event for watchpoint n is generated.
>
> That's all clear as mud for someone that don't speak arm. Can you please
> provide a coherent reason for all this that does not rely on external
> resources?

In short, when developing hardware watchpoint on LoongArch, we want to
set the same privilege passed by the ptrace user data, but there is no
a middle bridge to save this value like bp_addr, bp_type and bp_len, I
think this is a common issue for the archs which have privilege level
of breakpoint.

Thanks,
Tiezhu