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Message-ID: <20250626134158.3385080-4-glider@google.com> Date: Thu, 26 Jun 2025 15:41:50 +0200 From: Alexander Potapenko <glider@...gle.com> To: glider@...gle.com Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com, Aleksandr Nogikh <nogikh@...gle.com>, Andrey Konovalov <andreyknvl@...il.com>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, Dmitry Vyukov <dvyukov@...gle.com>, Ingo Molnar <mingo@...hat.com>, Josh Poimboeuf <jpoimboe@...nel.org>, Marco Elver <elver@...gle.com>, Peter Zijlstra <peterz@...radead.org>, Thomas Gleixner <tglx@...utronix.de> Subject: [PATCH v2 03/11] kcov: elaborate on using the shared buffer Add a paragraph about the shared buffer usage to kcov.rst. Signed-off-by: Alexander Potapenko <glider@...gle.com> --- 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 ------------------------------ -- 2.50.0.727.gbf7dc18ff4-goog
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