Messages in this thread | | | Date | Mon, 28 Sep 2020 15:47:56 +0100 | From | Dave Martin <> | Subject | Re: [PATCH 5/5] perf: arm_spe: Decode SVE events |
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On Mon, Sep 28, 2020 at 02:59:34PM +0100, André Przywara wrote: > 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 ;-)
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 ...
> >> + 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.
Mostly I'm curious because the encoding doesn't match the SVE architecture: SVE requires 4 bits to specify the vector length, not 3. This might have been a deliberate limitation in the SPE spec., but it raises questions about what should happen when 3 bits is not enough.
For SVE, valid vector lengths are 16 bytes * n or equivalently 128 bits * n), where 1 <= n <= 16.
The code here though cannot print EVLEN16 or EVLEN48 etc. This might not be a bug, but I'd like to understand where it comes from...
> > > > > 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
Fair enough.
Cheers ---Dave
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