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

[Leo Yan]

Hi Dave,

On Wed, Sep 30, 2020 at 11:34:11AM +0100, Dave Martin wrote:

[...]

> > > > >> 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 ;-)
> > > 
> > > It's probably not worth losing sleep over.
> > > 
> > > snprintf(3) says, under NOTES:
> > > 
> > > 	Until glibc 2.0.6, they would return -1 when the output was
> > > 	truncated.
> > > 
> > > which is probably ancient enough history that we don't care.  C11 does
> > > say that a negative return value can happen "if an encoding error
> > > occurred".  _Probably_ not a problem if perf tool never calls
> > > setlocale(), but ...
> > 
> > I have one patch which tried to fix the snprintf+increment sequence
> > [1], to be honest, the change seems urgly for me.  I agree it's better
> > to use a helper to wrap up.
> > 
> > [1] https://lore.kernel.org/patchwork/patch/1288410/
> 
> Sure, putting explicit checks all over the place makes a lot of noise in
> the code.
> 
> I was wondering whether something along the following lines would work:
> 
> 	/* ... */
> 
> 	if (payload & SVE_EVT_PKT_GEN_EXCEPTION)
> 		buf_appendf_err(&buf, &buf_len, &ret, " EXCEPTION-GEN");
> 	if (payload & SVE_EVT_PKT_ARCH_RETIRED)
> 		buf_appendf_err(&buf, &buf_len, &ret, " RETIRED");
> 	if (payload & SVE_EVT_PKT_L1D_ACCESS)
> 		buf_appendf_err(&buf, &buf_len, &ret, " L1D-ACCESS");
> 
> 	/* ... */
> 
> 	if (ret)
> 		return ret;
> 
> [...]

I have sent out the patch v2 [1] and Cc'ed you; I used a similiar API
definition with your suggestion:

  static int arm_spe_pkt_snprintf(char **buf_p, size_t *blen,
 				  const char *fmt, ...)

Only a difference is when return from arm_spe_pkt_snprintf(), will check
the return value and directly bail out when detect failure.  Your input
will be considered for next spin.

> Best to keep such refactoring independent of this series though.

Yeah, the patch set [2] is quite heavy; after get some reviewing,
maybe need to consider to split into 2 or even 3 small patch sets.

Thanks a lot for your suggestions!

Leo

[1] https://lore.kernel.org/patchwork/patch/1314603/
[2] https://lore.kernel.org/patchwork/cover/1314599/