Re: [PATCH 5/5] perf: arm_spe: Decode SVE events

[André Przywara]

On 28/09/2020 14:21, Dave Martin wrote:

Hi Dave,

> On Tue, Sep 22, 2020 at 11:12:25AM +0100, Andre Przywara wrote:
>> The Scalable Vector Extension (SVE) is an ARMv8 architecture extension
>> that introduces very long vector operations (up to 2048 bits).
> 
> (8192, in fact, though don't expect to see that on real hardware any
> time soon...  qemu and the Arm fast model can do it, though.)
> 
>> The SPE profiling feature can tag SVE instructions with additional
>> properties like predication or the effective vector length.
>>
>> Decode the new operation type bits in the SPE decoder to allow the perf
>> tool to correctly report about SVE instructions.
> 
> 
> I don't know anything about SPE, so just commenting on a few minor
> things that catch my eye here.

Many thanks for taking a look!
Please note that I actually missed a prior submission by Wei, so the
code changes here will end up in:
https://lore.kernel.org/patchwork/patch/1288413/

But your two points below magically apply to his patch as well, so....

> 
>> Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
>> ---
>>  .../arm-spe-decoder/arm-spe-pkt-decoder.c     | 48 ++++++++++++++++++-
>>  1 file changed, 47 insertions(+), 1 deletion(-)
>>
>> 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 a033f34846a6..f0c369259554 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
>> @@ -372,8 +372,35 @@ int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
>>  	}
>>  	case ARM_SPE_OP_TYPE:
>>  		switch (idx) {
>> -		case 0:	return snprintf(buf, buf_len, "%s", payload & 0x1 ?
>> +		case 0: {
>> +			size_t blen = buf_len;
>> +
>> +			if ((payload & 0x89) == 0x08) {
>> +				ret = snprintf(buf, buf_len, "SVE");
>> +				buf += ret;
>> +				blen -= ret;
> 
> (Nit: can ret be < 0 ?  I've never been 100% clear on this myself for
> the s*printf() family -- if this assumption is widespread in perf tool
> a lready that I guess just go with the flow.)

Yeah, some parts of the code in here check for -1, actually, but doing
this on every call to snprintf would push this current code over the
edge - and I cowardly avoided a refactoring ;-)

Please note that his is perf userland, and also we are printing constant
strings here.
Although admittedly this starts to sounds like an excuse now ...

> I wonder if this snprintf+increment+decrement sequence could be wrapped
> up as a helper, rather than having to be repeated all over the place.

Yes, I was hoping nobody would notice ;-)

>> +				if (payload & 0x2)
>> +					ret = snprintf(buf, buf_len, " FP");
>> +				else
>> +					ret = snprintf(buf, buf_len, " INT");
>> +				buf += ret;
>> +				blen -= ret;
>> +				if (payload & 0x4) {
>> +					ret = snprintf(buf, buf_len, " PRED");
>> +					buf += ret;
>> +					blen -= ret;
>> +				}
>> +				/* Bits [7..4] encode the vector length */
>> +				ret = snprintf(buf, buf_len, " EVLEN%d",
>> +					       32 << ((payload >> 4) & 0x7));
> 
> Isn't this just extracting 3 bits (0x7)? 

Ah, right, the comment is wrong. It's actually bits [6:4].

> And what unit are we aiming
> for here: is it the number of bytes per vector, or something else?  I'm
> confused by the fact that this will go up in steps of 32, which doesn't
> seem to match up to the architecure.

So this is how SPE encodes the effective vector length in its payload:
the format is described in section "D10.2.7 Operation Type packet" in a
(recent) ARMv8 ARM. I put the above statement in a C file and ran all
input values through it, it produced the exact *bit* length values as in
the spec.

Is there any particular pattern you are concerned about?
I admit this is somewhat hackish, I can do an extra function to put some
comments in there.

> 
> I notice that bit 7 has to be zero to get into this if() though.
> 
>> +				buf += ret;
>> +				blen -= ret;
>> +				return buf_len - blen;
>> +			}
>> +
>> +			return snprintf(buf, buf_len, "%s", payload & 0x1 ?
>>  					"COND-SELECT" : "INSN-OTHER");
>> +			}
>>  		case 1:	{
>>  			size_t blen = buf_len;
>>  
>> @@ -403,6 +430,25 @@ int arm_spe_pkt_desc(const struct arm_spe_pkt *packet, char *buf,
>>  				ret = snprintf(buf, buf_len, " NV-SYSREG");
>>  				buf += ret;
>>  				blen -= ret;
>> +			} else if ((payload & 0x0a) == 0x08) {
>> +				ret = snprintf(buf, buf_len, " SVE");
>> +				buf += ret;
>> +				blen -= ret;
>> +				if (payload & 0x4) {
>> +					ret = snprintf(buf, buf_len, " PRED");
>> +					buf += ret;
>> +					blen -= ret;
>> +				}
>> +				if (payload & 0x80) {
>> +					ret = snprintf(buf, buf_len, " SG");
>> +					buf += ret;
>> +					blen -= ret;
>> +				}
>> +				/* Bits [7..4] encode the vector length */
>> +				ret = snprintf(buf, buf_len, " EVLEN%d",
>> +					       32 << ((payload >> 4) & 0x7));
> 
> Same comment as above.  Maybe have a common helper for decoding the
> vector length bits so it can be fixed in a single place?

Yup. Although I wonder if this is the smallest of the problems with this
function going forward.

Cheers,
Andre

> 
>> +				buf += ret;
>> +				blen -= ret;
>>  			} else if (payload & 0x4) {
>>  				ret = snprintf(buf, buf_len, " SIMD-FP");
>>  				buf += ret;
> 
> Cheers
> ---Dave
>