docs: vmcoreinfo: add lockless printk ringbuffer vmcoreinfo

With the introduction of the lockless printk ringbuffer, the
VMCOREINFO relating to the kernel log buffer was changed. Update the
documentation to match those changes.

Fixes: 896fbe20b4 ("printk: use the lockless ringbuffer")
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: John Ogness <john.ogness@linutronix.de>
Reviewed-by: Petr Mladek <pmladek@suse.com>
Signed-off-by: Petr Mladek <pmladek@suse.com>
Link: https://lore.kernel.org/r/20200814213316.6394-1-john.ogness@linutronix.de

Documentation/admin-guide/kdump/vmcoreinfo.rst

@@ -184,50 +184,123 @@ from this.
 Free areas descriptor. User-space tools use this value to iterate the
 free_area ranges. MAX_ORDER is used by the zone buddy allocator.
 
-log_first_idx
+prb
--------------
+---
 
-Index of the first record stored in the buffer log_buf. Used by
+A pointer to the printk ringbuffer (struct printk_ringbuffer). This
-user-space tools to read the strings in the log_buf.
+may be pointing to the static boot ringbuffer or the dynamically
+allocated ringbuffer, depending on when the the core dump occurred.
+Used by user-space tools to read the active kernel log buffer.
 
-log_buf
+printk_rb_static
--------
+----------------
 
-Console output is written to the ring buffer log_buf at index
+A pointer to the static boot printk ringbuffer. If @prb has a
-log_first_idx. Used to get the kernel log.
+different value, this is useful for viewing the initial boot messages,
+which may have been overwritten in the dynamically allocated
+ringbuffer.
 
-log_buf_len
+clear_seq
------------
-
-log_buf's length.
-
-clear_idx
 ---------
 
-The index that the next printk() record to read after the last clear
+The sequence number of the printk() record after the last clear
-command. It indicates the first record after the last SYSLOG_ACTION
+command. It indicates the first record after the last
-_CLEAR, like issued by 'dmesg -c'. Used by user-space tools to dump
+SYSLOG_ACTION_CLEAR, like issued by 'dmesg -c'. Used by user-space
-the dmesg log.
+tools to dump a subset of the dmesg log.
 
-log_next_idx
+printk_ringbuffer
-------------
+-----------------
 
-The index of the next record to store in the buffer log_buf. Used to
+The size of a printk_ringbuffer structure. This structure contains all
-compute the index of the current buffer position.
+information required for accessing the various components of the
+kernel log buffer.
 
-printk_log
+(printk_ringbuffer, desc_ring|text_data_ring|dict_data_ring|fail)
-----------
+-----------------------------------------------------------------
 
-The size of a structure printk_log. Used to compute the size of
+Offsets for the various components of the printk ringbuffer. Used by
-messages, and extract dmesg log. It encapsulates header information for
+user-space tools to view the kernel log buffer without requiring the
-log_buf, such as timestamp, syslog level, etc.
+declaration of the structure.
 
-(printk_log, ts_nsec|len|text_len|dict_len)
+prb_desc_ring
--------------------------------------------
+-------------
 
-It represents field offsets in struct printk_log. User space tools
+The size of the prb_desc_ring structure. This structure contains
-parse it and check whether the values of printk_log's members have been
+information about the set of record descriptors.
-changed.
+
+(prb_desc_ring, count_bits|descs|head_id|tail_id)
+-------------------------------------------------
+
+Offsets for the fields describing the set of record descriptors. Used
+by user-space tools to be able to traverse the descriptors without
+requiring the declaration of the structure.
+
+prb_desc
+--------
+
+The size of the prb_desc structure. This structure contains
+information about a single record descriptor.
+
+(prb_desc, info|state_var|text_blk_lpos|dict_blk_lpos)
+------------------------------------------------------
+
+Offsets for the fields describing a record descriptors. Used by
+user-space tools to be able to read descriptors without requiring
+the declaration of the structure.
+
+prb_data_blk_lpos
+-----------------
+
+The size of the prb_data_blk_lpos structure. This structure contains
+information about where the text or dictionary data (data block) is
+located within the respective data ring.
+
+(prb_data_blk_lpos, begin|next)
+-------------------------------
+
+Offsets for the fields describing the location of a data block. Used
+by user-space tools to be able to locate data blocks without
+requiring the declaration of the structure.
+
+printk_info
+-----------
+
+The size of the printk_info structure. This structure contains all
+the meta-data for a record.
+
+(printk_info, seq|ts_nsec|text_len|dict_len|caller_id)
+------------------------------------------------------
+
+Offsets for the fields providing the meta-data for a record. Used by
+user-space tools to be able to read the information without requiring
+the declaration of the structure.
+
+prb_data_ring
+-------------
+
+The size of the prb_data_ring structure. This structure contains
+information about a set of data blocks.
+
+(prb_data_ring, size_bits|data|head_lpos|tail_lpos)
+---------------------------------------------------
+
+Offsets for the fields describing a set of data blocks. Used by
+user-space tools to be able to access the data blocks without
+requiring the declaration of the structure.
+
+atomic_long_t
+-------------
+
+The size of the atomic_long_t structure. Used by user-space tools to
+be able to copy the full structure, regardless of its
+architecture-specific implementation.
+
+(atomic_long_t, counter)
+------------------------
+
+Offset for the long value of an atomic_long_t variable. Used by
+user-space tools to access the long value without requiring the
+architecture-specific declaration.
 
 (free_area.free_list, MIGRATE_TYPES)
 ------------------------------------