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SubjectRe: [PATCH 03/16] perf: Add ability to dump user regs
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On Tue, Apr 17, 2012 at 1:17 PM, Jiri Olsa <jolsa@redhat.com> wrote:
> Add new attr->user_regs bitmap that lets a user choose a set
> of user registers to dump to the sample. The layout of this
> bitmap is described in asm/perf_regs.h for archs that
> support register dump.
>
> The perf syscall will fail if attr->user_regs is non zero.
>
> The register value here are those 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).
>
> This is going to be useful to bring Dwarf CFI based stack unwinding
> on top of samples.
>
> TODO handle compat tasks
>
> Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
> Signed-off-by: Jiri Olsa <jolsa@redhat.com>
> ---
>  include/linux/perf_event.h |    8 +++++
>  kernel/events/core.c       |   63 ++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 71 insertions(+), 0 deletions(-)
>
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index ddbb6a9..c63b807 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -272,6 +272,12 @@ struct perf_event_attr {
>                __u64           config2; /* extension of config1 */
>        };
>        __u64   branch_sample_type; /* enum branch_sample_type */
> +
> +       /*
> +        * Arch specific mask that defines a set of user regs to dump on
> +        * samples. See asm/perf_regs.h for details.
> +        */
> +       __u64                   user_regs;
>  };
>
Don't like the name of the field too much. You be more explicit.
Something like user_sample_regs.

>  /*
> @@ -608,6 +614,7 @@ struct perf_guest_info_callbacks {
>  #include <linux/atomic.h>
>  #include <linux/sysfs.h>
>  #include <asm/local.h>
> +#include <asm/perf_regs.h>
>
>  #define PERF_MAX_STACK_DEPTH           255
>
> @@ -1130,6 +1137,7 @@ struct perf_sample_data {
>        struct perf_callchain_entry     *callchain;
>        struct perf_raw_record          *raw;
>        struct perf_branch_stack        *br_stack;
> +       struct pt_regs                  *uregs;
>  };
>
>  static inline void perf_sample_data_init(struct perf_sample_data *data, u64 addr)
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index a6a9ec4..9f29fc3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3751,6 +3751,37 @@ int perf_unregister_guest_info_callbacks(struct perf_guest_info_callbacks *cbs)
>  }
>  EXPORT_SYMBOL_GPL(perf_unregister_guest_info_callbacks);
>
> +static void
> +perf_output_sample_regs(struct perf_output_handle *handle,
> +                       struct pt_regs *regs, u64 mask)
> +{
> +       int i = 0;
> +
> +       do {
> +               u64 val;
> +
> +               if (mask & 1) {
> +                       val = perf_reg_value(regs, i);
> +                       perf_output_put(handle, val);
> +               }
> +
> +               mask >>= 1;
> +               i++;
> +       } while (mask);
> +}
> +
> +static struct pt_regs *perf_sample_uregs(struct pt_regs *regs)
> +{
> +       if (!user_mode(regs)) {
> +               if (current->mm)
> +                       regs = task_pt_regs(current);
> +               else
> +                       regs = NULL;
> +       }
> +
> +       return regs;
> +}
> +
You are assuming the user app is running the same ABI than that of the
kernel. That's not correct on X86 and possibly other architectures.

>  static void __perf_event_header__init_id(struct perf_event_header *header,
>                                         struct perf_sample_data *data,
>                                         struct perf_event *event)
> @@ -4011,6 +4042,21 @@ void perf_output_sample(struct perf_output_handle *handle,
>                        perf_output_put(handle, nr);
>                }
>        }
> +
> +       if (event->attr.user_regs) {
> +               u64 id;
> +
> +               /*
> +                * If there are no regs to dump, notice it through a
> +                * PERF_REGS_VERSION_NONE version.
> +                */
> +               id = data->uregs ? perf_reg_version() : PERF_REGS_VERSION_NONE;
> +               perf_output_put(handle, id);
> +
> +               if (id)
> +                       perf_output_sample_regs(handle, data->uregs,
> +                                               event->attr.user_regs);
> +       }
>  }
>
>  void perf_prepare_sample(struct perf_event_header *header,
> @@ -4062,6 +4108,16 @@ void perf_prepare_sample(struct perf_event_header *header,
>                }
>                header->size += size;
>        }
> +
> +       if (event->attr.user_regs) {
> +               int size = sizeof(u64); /* the version size */
> +
> +               data->uregs = perf_sample_uregs(regs);
> +               if (data->uregs)
> +                       size += hweight64(event->attr.user_regs) * sizeof(u64);
> +
> +               header->size += size;
> +       }
>  }
>
>  static void perf_event_output(struct perf_event *event,
> @@ -6112,6 +6168,13 @@ static int perf_copy_attr(struct perf_event_attr __user *uattr,
>                        attr->branch_sample_type = mask;
>                }
>        }
> +
> +       /*
> +        * Don't let throught invalid register mask (i.e. the architecture
> +        * does not support register dump at all).
> +        */
> +       ret = perf_reg_validate(attr->user_regs);
> +
Again, how do you deal with 32-bit apps vs. 64-bit kernel?

>  out:
>        return ret;
>
> --
> 1.7.7.6
>
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