linux-kernel - Re: [PATCH v2 1/3] perf: Add perf_event_attr::bp

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Message-ID: <7522fc14-aacc-a8e3-3258-9064d7e2936f@loongson.cn>
Date: Sat, 6 Jul 2024 13:31:03 +0800
From: Tiezhu Yang <yangtiezhu@...ngson.cn>
To: Will Deacon <will@...nel.org>
Cc: Mark Rutland <mark.rutland@....com>, Russell King
 <linux@...linux.org.uk>, Catalin Marinas <catalin.marinas@....com>,
 Oleg Nesterov <oleg@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 Ingo Molnar <mingo@...hat.com>, Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, linux-arm-kernel@...ts.infradead.org,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] perf: Add perf_event_attr::bp_priv



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@...ngson.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.
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.

Thanks,
Tiezhu