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    SubjectRe: [PATCH 02/17] perf: Add ability to attach registers dump to sample
    On Wed, May 02, 2012 at 01:37:03PM +0200, Jiri Olsa wrote:
    > Introducing new sample_type bit PERF_SAMPLE_REGS. Once set,
    > the sample_regs value determines the kind of registers going
    > to be attached to the sample.
    >
    > Currently only user level registers are supported, specified by
    > PERF_SAMPLE_REGS_USER sample_regs value. Meaning the register
    > values of the user space context as it was before the user entered
    > the kernel for whatever reason (syscall, irq, exception, or a PMI
    > happening in userspace).
    >
    > When PERF_SAMPLE_REGS and PERF_SAMPLE_REGS_USER are set, the
    > sample_regs_user bitmap lets a user choose a set of registers
    > to dump for the sample. The layout of this bitmap is described
    > in asm/perf_regs.h for archs that support register dump.
    >
    > This is going to be useful to bring Dwarf CFI based stack
    > unwinding on top of samples.
    >
    > Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
    > Signed-off-by: Jiri Olsa <jolsa@redhat.com>
    > ---
    > include/linux/perf_event.h | 25 ++++++++++-
    > kernel/events/core.c | 98 ++++++++++++++++++++++++++++++++++++++++++++
    > 2 files changed, 121 insertions(+), 2 deletions(-)
    >
    > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
    > index ddbb6a9..6a8c974 100644
    > --- a/include/linux/perf_event.h
    > +++ b/include/linux/perf_event.h
    > @@ -130,8 +130,9 @@ enum perf_event_sample_format {
    > PERF_SAMPLE_STREAM_ID = 1U << 9,
    > PERF_SAMPLE_RAW = 1U << 10,
    > PERF_SAMPLE_BRANCH_STACK = 1U << 11,
    > + PERF_SAMPLE_REGS = 1U << 12,

    Given that we have to provide a reg mask anyway, is this sample type
    necessary? Zeroed mask means we don't want to record regs.

    >
    > - PERF_SAMPLE_MAX = 1U << 12, /* non-ABI */
    > + PERF_SAMPLE_MAX = 1U << 13, /* non-ABI */
    > };
    >
    > /*
    > @@ -163,6 +164,15 @@ enum perf_branch_sample_type {
    > PERF_SAMPLE_BRANCH_HV)
    >
    > /*
    > + * Values for sample_regs when PERF_SAMPLE_REGS is set.
    > + * Defines register set to be attached to the sample.
    > + */
    > +enum perf_sample_regs {
    > + PERF_SAMPLE_REGS_USER = 1U << 0, /* user registers */
    > + PERF_SAMPLE_REGS_MAX = 1U << 1, /* non-ABI */
    > +};

    If this is an enum, we won't be able to record multiple types of regs
    (eg: user regs and event live regs) in a single sample.

    > +
    > +/*
    > * The format of the data returned by read() on a perf event fd,
    > * as specified by attr.read_format:
    > *
    > @@ -271,7 +281,16 @@ struct perf_event_attr {
    > __u64 bp_len;
    > __u64 config2; /* extension of config1 */
    > };
    > - __u64 branch_sample_type; /* enum branch_sample_type */
    > + __u64 branch_sample_type; /* enum perf_branch_sample_type */
    > +
    > + __u64 sample_regs; /* enum perf_sample_regs */
    > +
    > + /*
    > + * Arch specific mask for PERF_SAMPLE_REGS_USER setup.
    > + * Defines set of user regs to dump on samples.
    > + * See asm/perf_regs.h for details.
    > + */
    > + __u64 sample_regs_user;

    We need to sort that out now. How do we handle normal/compat regs?
    Do we want sample_regs_user_64 and sample_regs_user_32? Or can we
    make it possible with a single field like above? Should we
    expect more modes than just a simple native and compat? I don't
    know much the world outside x86.

    Would be nice to have the opinion of other people on this. Ingo, Peter, others?

    Thanks.


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