Re: [PATCH] perf/x86/intel/lbr: fix branch type encoding

[Stephane Eranian]

On Thu, Aug 11, 2022 at 3:23 PM Liang, Kan <kan.liang@xxxxxxxxxxxxxxx> wrote:
>
>
>
> On 2022-08-10 5:06 p.m., Stephane Eranian wrote:
> > With architected LBR, the procesosr can record the type of each sampled taken
> > branch. The type is encoded in 4-bit field in the LBR_INFO MSR of each entry.
> >
> > The branch type must then extracted and saved in the perf_branch_entry in the
> > perf_events sampling buffer. With the current code, the raw Intel encoding of
> > the branch is exported to user tools.
>
> In the intel_pmu_lbr_filter(), the raw encoding will be converted into
> the X86_BR_* format via arch_lbr_br_type_map[]. Then the
> common_branch_type() will convert the X86_BR_* format to the generic
> PERF_BR_* type and expose to user tools.
>
> I double check the existing arch_lbr_br_type_map[] and branch_map[].
> They should generate the same PERF_BR_* type as your arch_lbr_type_map[].
>
> Is there a test case which I can use to reproduce the problem?
>
I was doing a simple:
$ perf record -b -e cpu/event=0xc4/ ....
$ perf report -D
Looking at the LBR information and the BR type, many entries has no branch type.
What I see is a function where you do: e->type = get_lbr_br_type() and
that is what
is then saved in the buffer. Unless I am missing a later patch.


> Thanks,
> Kan
>
> > Yet tools, such as perf, expected the
> > branch type to be encoded using perf_events branch type enum
> > (see tools/perf/util/branch.c). As a result of the discrepancy, the output of
> > perf report -D shows bogus branch types.
> >
> > Fix the problem by converting the Intel raw encoding into the perf_events
> > branch type enum values. With that in place and with no changes to the tools,
> > the branch types are now reported properly.
> >
> > Signed-off-by: Stephane Eranian <eranian@xxxxxxxxxx>
> > ---
> >  arch/x86/events/intel/lbr.c | 35 ++++++++++++++++++++++++++++++++---
> >  1 file changed, 32 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
> > index 4f70fb6c2c1e..ef63d4d46b50 100644
> > --- a/arch/x86/events/intel/lbr.c
> > +++ b/arch/x86/events/intel/lbr.c
> > @@ -894,9 +894,23 @@ static DEFINE_STATIC_KEY_FALSE(x86_lbr_mispred);
> >  static DEFINE_STATIC_KEY_FALSE(x86_lbr_cycles);
> >  static DEFINE_STATIC_KEY_FALSE(x86_lbr_type);
> >
> > -static __always_inline int get_lbr_br_type(u64 info)
> > +/*
> > + * Array index encodes IA32_LBR_x_INFO Branch Type Encodings
> > + * as per Intel SDM Vol3b Branch Types section
> > + */
> > +static const int arch_lbr_type_map[]={
> > +     [0] = PERF_BR_COND,
> > +     [1] = PERF_BR_IND,
> > +     [2] = PERF_BR_UNCOND,
> > +     [3] = PERF_BR_IND_CALL,
> > +     [4] = PERF_BR_CALL,
> > +     [5] = PERF_BR_RET,
> > +};
> > +#define ARCH_LBR_TYPE_COUNT ARRAY_SIZE(arch_lbr_type_map)
> > +
> > +static __always_inline u16 get_lbr_br_type(u64 info)
> >  {
> > -     int type = 0;
> > +     u16 type = 0;
> >
> >       if (static_branch_likely(&x86_lbr_type))
> >               type = (info & LBR_INFO_BR_TYPE) >> LBR_INFO_BR_TYPE_OFFSET;
> > @@ -904,6 +918,21 @@ static __always_inline int get_lbr_br_type(u64 info)
> >       return type;
> >  }
> >
> > +/*
> > + * The kernel cannot expose raw Intel branch type encodings because they are
> > + * not generic. Instead, the function below  maps the encoding to the
> > + * perf_events user visible branch types.
> > + */
> > +static __always_inline int get_lbr_br_type_mapping(u64 info)
> > +{
> > +     if (static_branch_likely(&x86_lbr_type)) {
> > +             u16 raw_type = get_lbr_br_type(info);
> > +             if (raw_type < ARCH_LBR_TYPE_COUNT)
> > +                     return arch_lbr_type_map[raw_type];
> > +     }
> > +     return PERF_BR_UNKNOWN;
> > +}
> > +
> >  static __always_inline bool get_lbr_mispred(u64 info)
> >  {
> >       bool mispred = 0;
> > @@ -957,7 +986,7 @@ static void intel_pmu_store_lbr(struct cpu_hw_events *cpuc,
> >               e->in_tx        = !!(info & LBR_INFO_IN_TX);
> >               e->abort        = !!(info & LBR_INFO_ABORT);
> >               e->cycles       = get_lbr_cycles(info);
> > -             e->type         = get_lbr_br_type(info);
> > +             e->type         = get_lbr_br_type_mapping(info);
> >       }
> >
> >       cpuc->lbr_stack.nr = i;