linux-kernel - Re: [PATCH v2 14/14] perf arm-spe: Add support for ARMv8.3-SPE

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Message-ID: <df1faa9b-d8cb-ebcf-b70f-3672a6d8db1f@arm.com>
Date:   Wed, 21 Oct 2020 10:26:07 +0100
From:   André Przywara <andre.przywara@....com>
To:     Leo Yan <leo.yan@...aro.org>
Cc:     Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Wei Li <liwei391@...wei.com>,
        James Clark <james.clark@....com>,
        Dave Martin <Dave.Martin@....com>,
        linux-kernel@...r.kernel.org, Al Grant <Al.Grant@....com>
Subject: Re: [PATCH v2 14/14] perf arm-spe: Add support for ARMv8.3-SPE

On 21/10/2020 06:10, Leo Yan wrote:

Hi,

> On Tue, Oct 20, 2020 at 10:54:44PM +0100, Andrï¿½ Przywara wrote:
>> On 29/09/2020 14:39, Leo Yan wrote:
>>
>> Hi,
>>
>>> From: Wei Li <liwei391@...wei.com>
>>>
>>> This patch is to support Armv8.3 extension for SPE, it adds alignment
>>> field in the Events packet and it supports the Scalable Vector Extension
>>> (SVE) for Operation packet and Events packet with two additions:
>>>
>>>   - The vector length for SVE operations in the Operation Type packet;
>>>   - The incomplete predicate and empty predicate fields in the Events
>>>     packet.
>>>
>>> Signed-off-by: Wei Li <liwei391@...wei.com>
>>> Signed-off-by: Leo Yan <leo.yan@...aro.org>
>>> ---
>>>  .../arm-spe-decoder/arm-spe-pkt-decoder.c     | 84 ++++++++++++++++++-
>>>  .../arm-spe-decoder/arm-spe-pkt-decoder.h     |  6 ++
>>>  2 files changed, 87 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
>>> index 05a4c74399d7..3ec381fddfcb 100644
>>> --- a/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
>>> +++ b/tools/perf/util/arm-spe-decoder/arm-spe-pkt-decoder.c
>>> @@ -342,14 +342,73 @@ int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
>>>  					return ret;
>>>  			}
>>>  		}
>>> +		if (idx > 2) {
>>
>> As I mentioned in the other patch, I doubt this extra comparison is
>> useful. Does that protect us from anything?
> 
> It's the same reason with Event packet which have explained for replying
> patch 10, the condition is to respect the SPE specifiction:
> 
>   E[11], byte 1, bit [11], when SZ == 0b10 , or SZ == 0b11
>      Alignment.
>      ...
>      Otherwise this bit reads-as-zero.
> 
> So we gives higher priority for checking payload size than the Event
> bit setting; if you have other thinking for this, please let me know.

Ah, thanks for pointing this out. It looks like a bug in the manual
then, because I don't see why bit 11 should be any different from bits
[10:8] and bits [15:12] in this respect. And in the diagrams above you
clearly see bit 11 being shown even when SZ == 0b01.

I will try to follow this up here.

>>> +			if (payload & SPE_EVT_PKT_ALIGNMENT) {
>>
>> Mmh, but this is bit 11, right?
> 
> Yes.
> 
>> So would need to go into the (idx > 1)
>> section (covering bits 8-15)? Another reason to ditch this comparison above.
> 
> As has explained in patch 10, idx is not the same thing with "sz"
> field; "idx" stands for payload length in bytes, so:
> 
>   idx = 1 << sz
> 
> The spec defines the sz is 2 or 3, thus idx is 4 or 8; so this is why
> here use the condition "(idx > 2)".
> 
> I think here need to refine code for more explict expression so can
> avoid confusion.  So I think it's better to condition such like:
> 
>   if (payload_len >= 4) {

Yes, that would be (or have been) more helpful, but as mentioned in the
other patch, I'd rather see those comparisons go entirely.

Cheers,
Andre

> 
>      ...
> 
>   }
> 
>>> +				ret = snprintf(buf, buf_len, " ALIGNMENT");
>>> +				if (ret < 0)
>>> +					return ret;
>>> +				buf += ret;
>>> +				blen -= ret;
>>
>> Shouldn't we use the new arm_spe_pkt_snprintf() function here as well?
>> Or is there a reason that this doesn't work?
> 
> Goot point.  Will change to use arm_spe_pkt_snprintf().
> 
>>> +			}
>>> +			if (payload & SPE_EVT_PKT_SVE_PARTIAL_PREDICATE) {
>>> +				ret = snprintf(buf, buf_len, " SVE-PARTIAL-PRED");
>>> +				if (ret < 0)
>>> +					return ret;
>>> +				buf += ret;
>>> +				blen -= ret;
>>> +			}
>>> +			if (payload & SPE_EVT_PKT_SVE_EMPTY_PREDICATE) {
>>> +				ret = snprintf(buf, buf_len, " SVE-EMPTY-PRED");
>>> +				if (ret < 0)
>>> +					return ret;
>>> +				buf += ret;
>>> +				blen -= ret;
>>> +			}
>>> +		}
>>> +
>>>  		return buf_len - blen;
>>>  
>>>  	case ARM_SPE_OP_TYPE:
>>>  		switch (idx) {
>>>  		case SPE_OP_PKT_HDR_CLASS_OTHER:
>>> -			return arm_spe_pkt_snprintf(&buf, &blen,
>>> -					payload & SPE_OP_PKT_OTHER_SUBCLASS_COND ?
>>> -					"COND-SELECT" : "INSN-OTHER");
>>> +			if ((payload & SPE_OP_PKT_OTHER_SVE_SUBCLASS_MASK) ==
>>> +					SPE_OP_PKT_OTHER_SUBCLASS_SVG_OP) {
>>> +
>>> +				ret = arm_spe_pkt_snprintf(&buf, &blen, "SVE-OTHER");
>>> +				if (ret < 0)
>>> +					return ret;
>>> +
>>> +				/* Effective vector length: step is 32 bits */
>>> +				ret = arm_spe_pkt_snprintf(&buf, &blen, " EVLEN %d",
>>> +					32 << ((payload & SPE_OP_PKT_SVE_EVL_MASK) >>
>>> +						SPE_OP_PKT_SVE_EVL_SHIFT));
>>
>> Can you move this into a macro, and add a comment about how this works?
>> People might get confused over the "32 << something".
> 
> Yeah, will refine for it.
> 
> Thanks,
> Leo
>