Messages in this thread Patch in this message | | | Subject | Re: [PATCH 5/5] perf: arm_spe: Decode SVE events | From | André Przywara <> | Date | Mon, 28 Sep 2020 14:59:34 +0100 |
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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@arm.com> >> --- >> .../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 >
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