[PATCH v4 02/10] kcov: elaborate on using the shared buffer

[Alexander Potapenko <glider@google.com>]

Add a paragraph about the shared buffer usage to kcov.rst.

Signed-off-by: Alexander Potapenko <glider@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v3:
 - add Reviewed-by: Dmitry Vyukov

Change-Id: Ia47ef7c3fcc74789fe57a6e1d93e29a42dbc0a97
---
 Documentation/dev-tools/kcov.rst | 55 ++++++++++++++++++++++++++++++++
 1 file changed, 55 insertions(+)

diff --git a/Documentation/dev-tools/kcov.rst b/Documentation/dev-tools/kcov.rst
index 6611434e2dd24..abf3ad2e784e8 100644
--- a/Documentation/dev-tools/kcov.rst
+++ b/Documentation/dev-tools/kcov.rst
@@ -137,6 +137,61 @@ mmaps coverage buffer, and then forks child processes in a loop. The child
 processes only need to enable coverage (it gets disabled automatically when
 a thread exits).
 
+Shared buffer for coverage collection
+-------------------------------------
+KCOV employs a shared memory buffer as a central mechanism for efficient and
+direct transfer of code coverage information between the kernel and userspace
+applications.
+
+Calling ``ioctl(fd, KCOV_INIT_TRACE, size)`` initializes coverage collection for
+the current thread associated with the file descriptor ``fd``. The buffer
+allocated will hold ``size`` unsigned long values, as interpreted by the kernel.
+Notably, even in a 32-bit userspace program on a 64-bit kernel, each entry will
+occupy 64 bits.
+
+Following initialization, the actual shared memory buffer is created using::
+
+    mmap(NULL, size * sizeof(unsigned long), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0)
+
+The size of this memory mapping, calculated as ``size * sizeof(unsigned long)``,
+must be a multiple of ``PAGE_SIZE``.
+
+This buffer is then shared between the kernel and the userspace. The first
+element of the buffer contains the number of PCs stored in it.
+Both the userspace and the kernel may write to the shared buffer, so to avoid
+race conditions each userspace thread should only update its own buffer.
+
+Normally the shared buffer is used as follows::
+
+              Userspace                                         Kernel
+    -----------------------------------------+-------------------------------------------
+    ioctl(fd, KCOV_INIT_TRACE, size)         |
+                                             |    Initialize coverage for current thread
+    mmap(..., MAP_SHARED, fd, 0)             |
+                                             |    Allocate the buffer, initialize it
+                                             |    with zeroes
+    ioctl(fd, KCOV_ENABLE, KCOV_TRACE_PC)    |
+                                             |    Enable PC collection for current thread
+                                             |    starting at buffer[1] (KCOV_ENABLE will
+                                             |    already write some coverage)
+    Atomically write 0 to buffer[0] to       |
+    reset the coverage                       |
+                                             |
+    Execute some syscall(s)                  |
+                                             |    Write new coverage starting at
+                                             |    buffer[1]
+    Atomically read buffer[0] to get the     |
+    total coverage size at this point in     |
+    time                                     |
+                                             |
+    ioctl(fd, KCOV_DISABLE, 0)               |
+                                             |    Write some more coverage for ioctl(),
+                                             |    then disable PC collection for current
+                                             |    thread
+    Safely read and process the coverage     |
+    up to the buffer[0] value saved above    |
+
+
 Comparison operands collection
 ------------------------------
 
-- 
2.50.1.552.g942d659e1b-goog