linux-kernel - Re: [PATCH v1 01/13] perf/core: add union to struct perf_branch

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Date:   Sat, 11 Sep 2021 00:16:20 +1000
From:   Michael Ellerman <mpe@...erman.id.au>
To:     Peter Zijlstra <peterz@...radead.org>,
        Stephane Eranian <eranian@...gle.com>
Cc:     linux-kernel@...r.kernel.org, acme@...hat.com, jolsa@...hat.com,
        kim.phillips@....com, namhyung@...nel.org, irogers@...gle.com,
        atrajeev@...ux.vnet.ibm.com, maddy@...ux.ibm.com
Subject: Re: [PATCH v1 01/13] perf/core: add union to struct perf_branch_entry

Michael Ellerman <mpe@...erman.id.au> writes:
> Peter Zijlstra <peterz@...radead.org> writes:
>> On Thu, Sep 09, 2021 at 12:56:48AM -0700, Stephane Eranian wrote:
>>> diff --git a/include/uapi/linux/perf_event.h b/include/uapi/linux/perf_event.h
>>> index f92880a15645..eb11f383f4be 100644
>>> --- a/include/uapi/linux/perf_event.h
>>> +++ b/include/uapi/linux/perf_event.h
>>> @@ -1329,13 +1329,18 @@ union perf_mem_data_src {
>>>  struct perf_branch_entry {
>>>  	__u64	from;
>>>  	__u64	to;
>>> -	__u64	mispred:1,  /* target mispredicted */
>>> -		predicted:1,/* target predicted */
>>> -		in_tx:1,    /* in transaction */
>>> -		abort:1,    /* transaction abort */
>>> -		cycles:16,  /* cycle count to last branch */
>>> -		type:4,     /* branch type */
>>> -		reserved:40;
>>> +	union {
>>> +		__u64	val;	    /* to make it easier to clear all fields */
>>> +		struct {
>>> +			__u64	mispred:1,  /* target mispredicted */
>>> +				predicted:1,/* target predicted */
>>> +				in_tx:1,    /* in transaction */
>>> +				abort:1,    /* transaction abort */
>>> +				cycles:16,  /* cycle count to last branch */
>>> +				type:4,     /* branch type */
>>> +				reserved:40;
>>> +		};
>>> +	};
>>>  };
>>
>>
>> Hurpmh... all other bitfields have ENDIAN_BITFIELD things except this
>> one. Power folks, could you please have a look?
>
> The bit number of each field changes between big and little endian, but
> as long as kernel and userspace are the same endian, and both only
> access values via the bitfields then it works.
...
>
> It does look like we have a bug in perf tool though, if I take a
> perf.data from a big endian system to a little endian one I don't see
> any of the branch flags decoded. eg:
>
> BE:
>
> 2413132652524 0x1db8 [0x2d0]: PERF_RECORD_SAMPLE(IP, 0x1): 5279/5279: 0xc00000000045c028 period: 923003 addr: 0
> ... branch stack: nr:28
> .....  0: c00000000045c028 -> c00000000dce7604 0 cycles  P   0
>
> LE:
>
> 2413132652524 0x1db8 [0x2d0]: PERF_RECORD_SAMPLE(IP, 0x1): 5279/5279: 0xc00000000045c028 period: 923003 addr: 0
> ... branch stack: nr:28
> .....  0: c00000000045c028 -> c00000000dce7604 0 cycles      0
>                                                          ^
>                                                          missing P
>
> I guess we're missing a byte swap somewhere.

Ugh. We _do_ have a byte swap, but we also need a bit swap.

That works for the single bit fields, not sure if it will for the
multi-bit fields.

So that's a bit of a mess :/

cheers