Re: [PATCH v1 1/9] perf: arm_spe: Introduce 'lds' capacity

[Will Deacon <will@kernel.org>]

On Fri, Aug 30, 2024 at 01:12:33PM +0100, Leo Yan wrote:
> On 8/30/24 11:38, Will Deacon wrote:
> > On Tue, Aug 27, 2024 at 05:44:09PM +0100, Leo Yan wrote:
> > > This commit adds a new entry 'lds' in the capacity folder. 'lds' stands
> > > for "loaded data source". When its value is 1, it indicates the data
> > > source implemented, and data source packets will be recorded in the
> > > trace data.
> > > 
> > > Signed-off-by: Leo Yan <leo.yan@arm.com>
> > > ---
> > >   drivers/perf/arm_spe_pmu.c | 3 +++
> > >   1 file changed, 3 insertions(+)
> > > 
> > > diff --git a/drivers/perf/arm_spe_pmu.c b/drivers/perf/arm_spe_pmu.c
> > > index 9100d82bfabc..81c1e7627721 100644
> > > --- a/drivers/perf/arm_spe_pmu.c
> > > +++ b/drivers/perf/arm_spe_pmu.c
> > > @@ -110,6 +110,7 @@ enum arm_spe_pmu_buf_fault_action {
> > >   /* This sysfs gunk was really good fun to write. */
> > >   enum arm_spe_pmu_capabilities {
> > >        SPE_PMU_CAP_ARCH_INST = 0,
> > > +     SPE_PMU_CAP_LDS,
> > >        SPE_PMU_CAP_ERND,
> > >        SPE_PMU_CAP_FEAT_MAX,
> > >        SPE_PMU_CAP_CNT_SZ = SPE_PMU_CAP_FEAT_MAX,
> > > @@ -118,6 +119,7 @@ enum arm_spe_pmu_capabilities {
> > > 
> > >   static int arm_spe_pmu_feat_caps[SPE_PMU_CAP_FEAT_MAX] = {
> > >        [SPE_PMU_CAP_ARCH_INST] = SPE_PMU_FEAT_ARCH_INST,
> > > +     [SPE_PMU_CAP_LDS]       = SPE_PMU_FEAT_LDS,
> > >        [SPE_PMU_CAP_ERND]      = SPE_PMU_FEAT_ERND,
> > >   };
> > > 
> > > @@ -160,6 +162,7 @@ static ssize_t arm_spe_pmu_cap_show(struct device *dev,
> > > 
> > >   static struct attribute *arm_spe_pmu_cap_attr[] = {
> > >        SPE_CAP_EXT_ATTR_ENTRY(arch_inst, SPE_PMU_CAP_ARCH_INST),
> > > +     SPE_CAP_EXT_ATTR_ENTRY(lds, SPE_PMU_CAP_LDS),
> > >        SPE_CAP_EXT_ATTR_ENTRY(ernd, SPE_PMU_CAP_ERND),
> > >        SPE_CAP_EXT_ATTR_ENTRY(count_size, SPE_PMU_CAP_CNT_SZ),
> > >        SPE_CAP_EXT_ATTR_ENTRY(min_interval, SPE_PMU_CAP_MIN_IVAL),
> > 
> > What will userspace do with this? I don't think you can turn LDS on/off,
> > so either you'll get the data source packet or you won't.
> 
> Yes, LDS bit does not work as a switch.
> 
> The tool in the userspace will record the LDS bit into the metadata. During
> decoding phase, it reads out the LDS from metadata. Based on it, the perf
> tool can know if the data source is supported or not, if yes then decode the
> data source packet.

Why not just decode a data source packet when you see it? i.e. assume LDS
is always set.

> Another point is how to decide the data source packet format. Now we maintain
> a CPU list for tracking CPU variants which support data source trace. For long
> term, I would like the tool can based on hardware feature (e.g. a ID register
> in Arm SPE) to decide the data source format, so far it is absent. This is why
> LDS bit + CPU list is a more reliable way. See some discussion [1].

Huh. Why would you have a CPU in the list if it _doesn't_ have LDS? If
we have to resort to per-CPU decoding, then that's even more of a reason
not to have the LDS cap imo.

Will