Date:	Fri, 23 Aug 2013 23:00:15 +0200
From:	Frantisek Hrbata <fhrbata@...hat.com>
To:	Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
Cc:	linux-kernel@...r.kernel.org, jstancek@...hat.com,
	keescook@...omium.org, rusty@...tcorp.com.au,
	linux-arch@...r.kernel.org, arnd@...db.de, mgahagan@...hat.com,
	agospoda@...hat.com
Subject: Re: [RFC PATCH 2/4] gcov: add support for gcc 4.7 gcov format

On Fri, Aug 23, 2013 at 05:12:19PM +0200, Peter Oberparleiter wrote:
> On 23.08.2013 10:39, Frantisek Hrbata wrote:
> > The gcov in-memory format changed in gcc 4.7. The biggest change, which
> > requires this special implementation, is that gcov_info no longer contains
> > array of counters for each counter type for all functions and gcov_fn_info is
> > not used for mapping of function's counters to these arrays(offset). Now each
> > gcov_fn_info contans it's counters, which makes things a little bit easier.
> 
> By now I've come to the conclusion that I may have over-engineered
> gcov_iter_next in the original GCC 3.4 implementation. This became especially
> apparent when someone tried to adjust that code to also work with a specific
> android GCC version - adding a simple field to the output file format required
> major complex changes.
> 
> Therefore in my version of the GCC 4.7 support, I opted to replace this logic
> with the simpler approach of generating the gcov data file in-memory during
> open (gcov_iter_new) and reducing gcov_iter_next/gcov_iter_write to simple
> copy-from-buffer functions.

Yes, this seems reasonable and much easier approach to me, but we will end up
with three copies of one gcov(gcda) file data in memory: data from gcc, our
buffer and the buffer in seq file. But I guess this is not a big problem,
unless someone opens a huge amount of gcda files in parallel. Generally I
like this idea. This will be also probably faster then writing 1 or 2 ints per
one iter write.

> 
> Since I can see the need to change the format code again in the future,
> I think it would be easier to simplify this part of the code correspondingly.
> I'm adding my version of this part of the implementation as discussion input
> below (the relevant part starts at convert_to_gcda()).

I have really only two nits to the code(please see below). I spent some time
checking the new seq/iter implementation and it seems ok to me. Generally I like
how simple this change is done.

> 
> ---
> kernel: gcov: add support for gcc 4.7 gcov format
> 
> GCC 4.7 changed the format of gcov related data structures, both
> on-disk and in memory. Add support for the new format.
> 
> Signed-off-by: Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
> ---
>  kernel/gcov/Makefile  |    4
>  kernel/gcov/gcc_4_7.c |  510 ++++++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 513 insertions(+), 1 deletion(-)
> 
> --- a/kernel/gcov/Makefile
> +++ b/kernel/gcov/Makefile
> @@ -1,3 +1,5 @@
>  ccflags-y := -DSRCTREE='"$(srctree)"' -DOBJTREE='"$(objtree)"'
> 
> -obj-$(CONFIG_GCOV_KERNEL) := base.o fs.o gcc_3_4.o
> +obj-$(CONFIG_GCOV_KERNEL) := base.o fs.o
> +obj-$(CONFIG_GCOV_KERNEL) += $(call cc-ifversion, -lt, 0407, gcc_3_4.o)
> +obj-$(CONFIG_GCOV_KERNEL) += $(call cc-ifversion, -ge, 0407, gcc_4_7.o)
> --- /dev/null
> +++ b/kernel/gcov/gcc_4_7.c
> @@ -0,0 +1,510 @@
> +/*
> + *  This code provides functions to handle gcc's profiling data format
> + *  introduced with gcc 4.7. Future versions of gcc may change the gcov
> + *  format (as happened before), so all format-specific information needs
> + *  to be kept modular and easily exchangeable.
> + *
> + *  This file is based on gcc-internal definitions. Functions and data
> + *  structures are defined to be compatible with gcc counterparts.
> + *  For a better understanding, refer to gcc source: gcc/gcov-io.h.
> + *
> + *    Copyright IBM Corp. 2013
> + *    Author(s): Peter Oberparleiter <oberpar@...ux.vnet.ibm.com>
> + *
> + *    Uses gcc-internal data definitions.
> + */
> +
> +#include <linux/errno.h>
> +#include <linux/slab.h>
> +#include <linux/string.h>
> +#include <linux/seq_file.h>
> +#include <linux/vmalloc.h>
> +#include <linux/types.h>
> +#include "gcov.h"
> +
> +/*
> + * Profiling data types used for gcc 4.7 and above - these are defined by
> + * gcc and need to be kept as close to the original definition as possible to
> + * remain compatible.
> + */
> +#define GCOV_COUNTERS		8
> +#define GCOV_DATA_MAGIC		((unsigned int) 0x67636461)
> +#define GCOV_TAG_FUNCTION	((unsigned int) 0x01000000)
> +#define GCOV_TAG_COUNTER_BASE	((unsigned int) 0x01a10000)
> +#define GCOV_TAG_FOR_COUNTER(count)					\
> +	(GCOV_TAG_COUNTER_BASE + ((unsigned int) (count) << 17))
> +
> +#if BITS_PER_LONG >= 64
> +typedef long gcov_type;
> +#else
> +typedef long long gcov_type;
> +#endif
> +
> +/**
> + * struct gcov_ctr_info - profiling data per function and counter type
> + * @num: number of counter values for this type
> + * @values: array of counter values for this type
> + *
> + * This data is generated by gcc during compilation and doesn't change
> + * at run-time with the exception of the values array.
> + */
> +struct gcov_ctr_info {
> +	unsigned int	num;
> +	gcov_type	*values;
> +};
> +
> +/**
> + * struct gcov_fn_info - profiling meta data per function
> + * @key: comdat key
> + * @ident: object file-unique function identifier
> + * @lineno_checksum: function lineno_checksum
> + * @cfg_checksum: function cfg_checksum
> + * @ctrs: counters
> + *
> + * This data is generated by gcc during compilation and doesn't change
> + * at run-time.
> + */
> +struct gcov_fn_info {
> +	const struct gcov_info *key;
> +	unsigned int ident;
> +	unsigned int lineno_checksum;
> +	unsigned int cfg_checksum;
> +	struct gcov_ctr_info ctrs[0];
> +};
> +
> +typedef void (*gcov_merge_fn)(gcov_type *, unsigned int);
> +
> +/**
> + * struct gcov_info - profiling data per object file
> + * @version: gcov version magic indicating the gcc version used for compilation
> + * @next: list head for a singly-linked list
> + * @stamp: time stamp
> + * @filename: name of the associated gcov data file
> + * @merge: functions for merging counts per counter type or null for unused
> + * @n_functions: number of instrumented functions
> + * @functions: function data
> + *
> + * This data is generated by gcc during compilation and doesn't change
> + * at run-time with the exception of the next pointer.
> + */
> +struct gcov_info {
> +	unsigned int			version;
> +	struct gcov_info		*next;
> +	unsigned int			stamp;
> +	const char			*filename;
> +	gcov_merge_fn			merge[GCOV_COUNTERS];
> +	unsigned int			n_functions;
> +	struct gcov_fn_info		**functions;

I can see that you removed the const part. I was thinking about it too, because
it requires ugly cast-const-away code in the gcov_info_dup and it is imho not
necessary, but I left the original definition from gcc anyway.

> +};
> +
> +/* Symbolic links to be created for each profiling data file. */
> +const struct gcov_link gcov_link[] = {
> +	{ OBJ_TREE, "gcno" },	/* Link to .gcno file in $(objtree). */
> +	{ 0, NULL},
> +};
> +
> +/*
> + * Determine whether a counter is active. Based on gcc magic. Doesn't change
> + * at run-time.
> + */
> +static int counter_active(struct gcov_info *info, unsigned int type)
> +{
> +	return info->merge[type] != NULL;
> +}
> +
> +/* Determine number of active counters. Based on gcc magic. */
> +static unsigned int num_counter_active(struct gcov_info *info)
> +{
> +	unsigned int i;
> +	unsigned int result = 0;
> +
> +	for (i = 0; i < GCOV_COUNTERS; i++) {
> +		if (counter_active(info, i))
> +			result++;
> +	}
> +	return result;
> +}
> +
> +/**
> + * gcov_info_reset - reset profiling data to zero
> + * @info: profiling data set
> + */
> +void gcov_info_reset(struct gcov_info *info)
> +{
> +	unsigned int active = num_counter_active(info);
> +	unsigned int f, i;
> +	struct gcov_ctr_info *ctr;
> +
> +	for (f = 0; f < info->n_functions; f++) {
> +		for (i = 0; i < active; i++) {
> +			ctr = &info->functions[f]->ctrs[i];
> +			memset(ctr->values, 0, ctr->num * sizeof(gcov_type));
> +		}
> +	}
> +}
> +
> +/**
> + * gcov_info_is_compatible - check if profiling data can be added
> + * @info1: first profiling data set
> + * @info2: second profiling data set
> + *
> + * Returns non-zero if profiling data can be added, zero otherwise.
> + */
> +int gcov_info_is_compatible(struct gcov_info *info1, struct gcov_info *info2)
> +{
> +	return (info1->stamp == info2->stamp);
> +}
> +
> +/**
> + * gcov_info_add - add up profiling data
> + * @dest: profiling data set to which data is added
> + * @source: profiling data set which is added
> + *
> + * Adds profiling counts of @source to @dest.
> + */
> +void gcov_info_add(struct gcov_info *dest, struct gcov_info *source)
> +{
> +	unsigned int active = num_counter_active(dest);
> +	unsigned int f, i, j;
> +
> +	for (f = 0; f < dest->n_functions; f++) {
> +		for (i = 0; i < active; i++) {
> +			struct gcov_ctr_info *d_ctr =
> +				&dest->functions[f]->ctrs[i];
> +			struct gcov_ctr_info *s_ctr =
> +				&source->functions[f]->ctrs[i];
> +
> +			for (j = 0; j < d_ctr->num; j++) {
> +				d_ctr->values[j] += s_ctr->values[j];
> +			}
> +		}
> +	}
> +}
> +
> +/**
> + * gcov_info_dup - duplicate profiling data set
> + * @info: profiling data set to duplicate
> + *
> + * Return newly allocated duplicate on success, %NULL on error.
> + */
> +struct gcov_info *gcov_info_dup(struct gcov_info *info)
> +{
> +	struct gcov_info *dup;
> +	unsigned int active = num_counter_active(info);
> +	unsigned int i, j;
> +
> +	/* Duplicate gcov_info. */
> +	dup = kzalloc(sizeof(struct gcov_info), GFP_KERNEL);
> +	if (!dup)
> +		return NULL;
> +	dup->version		= info->version;
> +	dup->stamp		= info->stamp;
> +	dup->n_functions	= info->n_functions;
> +	memcpy(dup->merge, info->merge, sizeof(dup->merge));
> +	/* Duplicate filename. */
> +	dup->filename		= kstrdup(info->filename, GFP_KERNEL);
> +	if (!dup->filename)
> +		goto err_free;
> +	/* Duplicate table of functions. */
> +	dup->functions = kzalloc(sizeof(struct gcov_fn_info *) *
> +				 info->n_functions, GFP_KERNEL);
> +	if (!dup->functions)
> +		goto err_free;
> +	for (i = 0; i < info->n_functions; i++) {
> +		struct gcov_fn_info *src_fn = info->functions[i];
> +		struct gcov_fn_info *dest_fn;
> +
> +		/* Duplicate gcov_fn_info. */
> +		dup->functions[i] = kzalloc(sizeof(struct gcov_fn_info) +
> +					    sizeof(struct gcov_ctr_info) *
> +					    active, GFP_KERNEL);
> +		if (!dup->functions[i])
> +			goto err_free;
> +		dest_fn = dup->functions[i];
> +		dest_fn->ident			= src_fn->ident;
> +		dest_fn->lineno_checksum	= src_fn->lineno_checksum;
> +		dest_fn->cfg_checksum		= src_fn->cfg_checksum;
> +
> +		for (j = 0; j < active; j++) {
> +			struct gcov_ctr_info *src_ctr = &src_fn->ctrs[j];
> +			struct gcov_ctr_info *dest_ctr = &dest_fn->ctrs[j];
> +			size_t size = sizeof(gcov_type) * src_ctr->num;
> +
> +			/* Duplicate gcov_ctr_info. */
> +			dest_ctr->num		= src_ctr->num;
> +			dest_ctr->values	= vmalloc(size);
                                                  ^^^^^^^
Does the vmalloc make sense here? The counters are allocated per function, so I
expect they will be pretty small compared when there was one "huge" array for
each counter type for all functions per gcov_info. Meaning here we will not
consume a "big" continuous block of phys memory. I'm just currious and maybe I'm
missing something. Anyway this is just a nit.

> +			if (!dest_ctr->values)
> +				goto err_free;
> +			memcpy(dest_ctr->values, src_ctr->values, size);
> +		}
> +	}
> +	return dup;
> +
> +err_free:
> +	gcov_info_free(dup);
> +	return NULL;
> +}
> +
> +/**
> + * gcov_info_free - release memory for profiling data set duplicate
> + * @info: profiling data set duplicate to free
> + */
> +void gcov_info_free(struct gcov_info *info)
> +{
> +	unsigned int active = num_counter_active(info);
> +	unsigned int i, j;
> +
> +	if (info->functions) {
> +		for (i = 0; i < info->n_functions; i++) {
> +			if (!info->functions[i])
> +				continue;
> +			for (j = 0; j < active; j++)
> +				vfree(info->functions[i]->ctrs[j].values);
> +			kfree(info->functions[i]);
> +		}
> +	}
> +	kfree(info->functions);
> +	kfree(info->filename);
> +	kfree(info);
> +}
> +
> +#define ITER_STRIDE	PAGE_SIZE
> +
> +/**
> + * struct gcov_iterator - specifies current file position in logical records
> + * @buffer: buffer containing file data
> + * @size: size of buffer
> + * @pos: current position in file
> + */
> +struct gcov_iterator {
> +	struct gcov_info *info;
> +	void *buffer;
> +	size_t size;
> +	loff_t pos;
> +};
> +
> +/**
> + * store_gcov_u32 - store 32 bit number in gcov format to buffer
> + * @buffer: target buffer or NULL
> + * @off: offset into the buffer
> + * @v: value to be stored
> + *
> + * Number format defined by gcc: numbers are recorded in the 32 bit
> + * unsigned binary form of the endianness of the machine generating the
> + * file. Returns the number of bytes stored. If @buffer is %NULL, doesn't
> + * store anything.
> + */
> +static size_t store_gcov_u32(void *buffer, size_t off, u32 v)
> +{
> +	u32 *data;
> +
> +	if (buffer) {
> +		data = buffer + off;
> +		*data = v;
> +	}
> +
> +	return sizeof(*data);
> +}
> +
> +/**
> + * store_gcov_u64 - store 64 bit number in gcov format to buffer
> + * @buffer: target buffer or NULL
> + * @off: offset into the buffer
> + * @v: value to be stored
> + *
> + * Number format defined by gcc: numbers are recorded in the 32 bit
> + * unsigned binary form of the endianness of the machine generating the
> + * file. 64 bit numbers are stored as two 32 bit numbers, the low part
> + * first. Returns the number of bytes stored. If @buffer is %NULL, doesn't store
> + * anything.
> + */
> +static size_t store_gcov_u64(void *buffer, size_t off, u64 v)
> +{
> +	u32 *data;
> +
> +	if (buffer) {
> +		data = buffer + off;
> +
> +		data[0] = (v & 0xffffffffUL);
> +		data[1] = (v >> 32);
> +	}
> +
> +	return sizeof(*data) * 2;
> +}
> +
> +/**
> + * get_ctr_type - return type of specified counter
> + * @info: profiling data set
> + * @num: index into list of active counters
> + *
> + * Returns the type of the specified counter.
> + */
> +static int get_ctr_type(struct gcov_info *info, unsigned int num)
> +{
> +	unsigned int type;
> +
> +	for (type = 0; type < GCOV_COUNTERS; type++) {
> +		if (counter_active(info, type)) {
> +			if (num == 0)
> +				break;
> +			num--;
> +		}
> +	}
> +
> +	return type;
> +}
> +
> +/**
> + * convert_to_gcda - convert profiling data set to gcda file format
> + * @buffer: the buffer to store file data or %NULL if no data should be stored
> + * @info: profiling data set to be converted
> + *
> + * Returns the number of bytes that were/would have been stored into the buffer.
> + */
> +static size_t convert_to_gcda(char *buffer, struct gcov_info *info)
> +{
> +	unsigned int active = num_counter_active(info);
> +	unsigned int i, j, k;
> +	size_t pos = 0;
> +
> +	/* File header. */
> +	pos += store_gcov_u32(buffer, pos, GCOV_DATA_MAGIC);
> +	pos += store_gcov_u32(buffer, pos, info->version);
> +	pos += store_gcov_u32(buffer, pos, info->stamp);
> +
> +	for (i = 0; i < info->n_functions; i++) {
> +		struct gcov_fn_info *fn = info->functions[i];
> +
> +		/* Function record. */
> +		pos += store_gcov_u32(buffer, pos, GCOV_TAG_FUNCTION);
> +		pos += store_gcov_u32(buffer, pos, 3);
> +		pos += store_gcov_u32(buffer, pos, fn->ident);
> +		pos += store_gcov_u32(buffer, pos, fn->lineno_checksum);
> +		pos += store_gcov_u32(buffer, pos, fn->cfg_checksum);
> +
> +		for (j = 0; j < active; j++) {
> +			struct gcov_ctr_info *ctr = &fn->ctrs[j];
> +			int type = get_ctr_type(info, j);
> +
> +			/* Counter record. */
> +			pos += store_gcov_u32(buffer, pos,
> +					      GCOV_TAG_FOR_COUNTER(type));
> +			pos += store_gcov_u32(buffer, pos, ctr->num * 2);
> +
> +			for (k = 0; k < ctr->num; k++) {
> +				pos += store_gcov_u64(buffer, pos,
> +						      ctr->values[k]);
> +			}
> +		}
> +
> +
> +	}
> +
> +	return pos;
> +}
> +
> +/**
> + * gcov_iter_new - allocate and initialize profiling data iterator
> + * @info: profiling data set to be iterated
> + *
> + * Return file iterator on success, %NULL otherwise.
> + */
> +struct gcov_iterator *gcov_iter_new(struct gcov_info *info)
> +{
> +	struct gcov_iterator *iter;
> +
> +	iter = kzalloc(sizeof(struct gcov_iterator), GFP_KERNEL);
> +	if (!iter)
> +		goto err_free;
> +
> +	iter->info = info;
> +	/* Dry-run to get the actual buffer size. */
> +	iter->size = convert_to_gcda(NULL, info);
> +	iter->buffer = vmalloc(iter->size);
> +	if (!iter->buffer)
> +		goto err_free;
> +
> +	convert_to_gcda(iter->buffer, info);
> +
> +	return iter;
> +
> +err_free:
> +	kfree(iter);
> +	return NULL;
> +}
> +
> +/**
> + * gcov_iter_free - release memory for iterator
> + * @iter: file iterator to free
> + */
> +void gcov_iter_free(struct gcov_iterator *iter)
> +{
> +	vfree(iter->buffer);
> +	kfree(iter);
> +}
> +
> +/**
> + * gcov_iter_get_info - return profiling data set for given file iterator
> + * @iter: file iterator
> + */
> +struct gcov_info *gcov_iter_get_info(struct gcov_iterator *iter)
> +{
> +	return iter->info;
> +}
> +
> +/**
> + * gcov_iter_start - reset file iterator to starting position
> + * @iter: file iterator
> + */
> +void gcov_iter_start(struct gcov_iterator *iter)
> +{
> +	iter->pos = 0;
> +}
> +
> +/**
> + * gcov_iter_next - advance file iterator to next logical record
> + * @iter: file iterator
> + *
> + * Return zero if new position is valid, non-zero if iterator has reached end.
> + */
> +int gcov_iter_next(struct gcov_iterator *iter)
> +{
> +	if (iter->pos < iter->size)
> +		iter->pos += ITER_STRIDE;
> +	if (iter->pos >= iter->size)
> +		return -EINVAL;
> +
> +	return 0;
> +}
> +
> +/**
> + * gcov_iter_write - write data for current pos to seq_file
> + * @iter: file iterator
> + * @seq: seq_file handle
> + *
> + * Return zero on success, non-zero otherwise.
> + */
> +int gcov_iter_write(struct gcov_iterator *iter, struct seq_file *seq)
> +{
> +	size_t len;
> +
> +	if (iter->pos >= iter->size)
> +		return -EINVAL;
> +
> +	len = ITER_STRIDE;
> +	if (iter->pos + len > iter->size)
> +		len = iter->size - iter->pos;
> +
> +	seq_write(seq, iter->buffer + iter->pos, len);
> +
> +	return 0;
> +}
> +
> +/**
> + * gcov_info_get_filename - return name of the associated gcov data file
> + * @info: profiling data set
> + */
> +const char *gcov_info_get_filename(struct gcov_info *info)
> +{
> +	return info->filename;
> +}
> 
> -- 
> Peter Oberparleiter
> Linux on System z Development - IBM Germany
> 

-- 
Frantisek Hrbata
--
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