RE: [PATCH v2 03/13] btrace, linux: add perf event buffer abstraction

["Metzger, Markus T" <markus dot t dot metzger at intel dot com>]

> -----Original Message-----
> From: Pedro Alves [mailto:palves@redhat.com]
> Sent: Tuesday, January 27, 2015 12:18 PM
> To: Metzger, Markus T
> Cc: gdb-patches@sourceware.org
> Subject: Re: [PATCH v2 03/13] btrace, linux: add perf event buffer abstraction
> 
> On 11/20/2014 10:47 AM, Markus Metzger wrote:
> 
> > -  volatile struct perf_event_mmap_page *header;
> > +  struct perf_event_buffer *pevent;
> >    const uint8_t *begin, *end, *start;
> > -  unsigned long data_head, data_tail, retries = 5;
> > -  size_t buffer_size, size;
> > +  unsigned long long data_head, data_tail, buffer_size, size;
> > +  size_t retries = 5;
> 
> I notice that this is changing types from long to long long.
> Also, a bit odd that retries is now size_t, given it's just a counter.

I changed retries to unsigned int.

data_head and data_tail correspond to respective fields in
struct perf_event_mmap_page, a kernel data structure.
buffer_size corresponds to the data_size field in the same
struct.

I can't say why they had been unsigned long before.  It was likely
a bug.


> > +#if HAVE_LINUX_PERF_EVENT_H
> > +/* A Linux perf event buffer.  */
> > +struct perf_event_buffer
> > +{
> > +  /* The mapped memory.  */
> > +  const uint8_t *mem;
> > +
> > +  /* The size of the mapped memory in bytes.  */
> > +  unsigned long long size;
> > +
> > +  /* A pointer to the data_head field for this buffer. */
> > +  volatile unsigned long long *data_head;
> > +
> > +  /* The data_head value from the last read.  */
> > +  unsigned long long last_head;
> > +};
> 
> Isn't there a better type to use here instead of "long long"?
> Why not size_t, if a host buffer size; uint64_t, if it's a fixed
> buffer format, assuming 64-bit here; or ULONGEST, the widest
> integer we support?

I'm using the same data type as the respective fields in
struct perf_event_mmap_page  (after resolving a few typedefs).

I'm also fine to directly use __u64 or to use uint64_t.

Thanks,
Markus.

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