[PATCH v2 0/2] perf riscv: Implement rdtsc()

[PATCH v2 0/2] perf riscv: Implement rdtsc()

[Chen Pei]

tools/perf falls back to the __weak rdtsc() stub in tools/perf/util/tsc.c
on every architecture that does not override it, riscv included.  The
stub returns a constant 0, so anything that reaches for the counter gets
a value that means nothing and cannot tell it apart from a counter that
happens to be at 0.

The time CSR is a natural TSC equivalent on riscv: readable from user
mode, constant frequency, and the same counter that riscv's
arch_perf_update_userpage() feeds the user page conversion fields from,
ever since commit 83c5e13b8cbb ("riscv: Prepare for user-space perf
event mmap support").  This series registers that on the tools side.

The two patches are split so that the first stands on its own:

1. Implement rdtsc() over the time CSR for riscv64.

2. Have the build system say which architectures have a usable counter,
   instead of a list of compiler macros inside the test.

What this is worth beyond the subtest result:

1. The subtest is the only reader of those conversion fields riscv
   can run, so it guards arithmetic that has shipped unexercised;
   registering the counter at the same time leaves a future riscv
   auxtrace driver the reading and the conversion already in place.

2. The flag lets a caller ask whether the architecture has a counter
   before it uses the value, without keeping its own copy of a list
   that has to be edited every time an architecture grows an
   implementation.

Changes since v1:
- Define rdtsc() for RV64 only. RV32 now keeps the weak stub.
- Declare the capability through the build system rather than through a
  weak arch__rdtsc_supported() hook, following the PERF_HAVE_JITDUMP
  pattern, as Leo Yan suggested.

Tests:
======
- riscv64 QEMU virt guest with sscofpmf, comparing the build before the
  series with the build after it: before, the "TSC support" subtest skips
  and the conversion subtest fails while comparing against the stub's
  constant 0; after, both report Ok.
- riscv64 and riscv32 cross builds: the compile command of
  tests/perf-time-to-tsc.o carries -DHAVE_RDTSC on rv64 and not on
  rv32, so RV32 stays on the weak stub.
- x86_64 host and arm64 cross builds: -DHAVE_RDTSC is in effect for
  both, and on the host the "TSC support" subtest still reports Ok.

Chen Pei (2):
  perf riscv: Implement rdtsc()
  perf tsc: Declare TSC support from the build system

 tools/perf/Makefile.config          |  4 ++++
 tools/perf/arch/arm64/Makefile      |  1 +
 tools/perf/arch/riscv/Makefile      |  1 +
 tools/perf/arch/riscv/util/Build    |  1 +
 tools/perf/arch/riscv/util/tsc.c    | 27 +++++++++++++++++++++++++++
 tools/perf/arch/x86/Makefile        |  1 +
 tools/perf/tests/perf-time-to-tsc.c | 19 ++++---------------
 7 files changed, 39 insertions(+), 15 deletions(-)
 create mode 100644 tools/perf/arch/riscv/util/tsc.c

-- 
2.50.1

[PATCH v2 1/2] perf riscv: Implement rdtsc()

[Chen Pei]

Perf has no riscv implementation of rdtsc(), so it links against the weak
stub in util/tsc.c and every caller reads a constant 0 that cannot be
told apart from a counter standing still.

Implement rdtsc() over the time CSR with rdtime.  The time CSR is a TSC
equivalent on riscv: readable from user mode, constant frequency, and the
same counter riscv's arch_perf_update_userpage() builds the user page
conversion fields from, so the counter and perf time share a timebase.

Define it for RV64 only.  Reading the counter whole on RV32 takes time and
timeh in a loop, and no RV32 user in tools/perf needs it that much, so
RV32 keeps the weak stub.

Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
---
Changes in v2:
- Define rdtsc() for RV64 only.

 tools/perf/arch/riscv/util/Build |  1 +
 tools/perf/arch/riscv/util/tsc.c | 27 +++++++++++++++++++++++++++
 2 files changed, 28 insertions(+)
 create mode 100644 tools/perf/arch/riscv/util/tsc.c

diff --git a/tools/perf/arch/riscv/util/Build b/tools/perf/arch/riscv/util/Build
index 2328fb9a30a3..52cd6a481de8 100644
--- a/tools/perf/arch/riscv/util/Build
+++ b/tools/perf/arch/riscv/util/Build
@@ -1 +1,2 @@
 perf-util-y += header.o
+perf-util-y += tsc.o
diff --git a/tools/perf/arch/riscv/util/tsc.c b/tools/perf/arch/riscv/util/tsc.c
new file mode 100644
index 000000000000..3dd419ce9d2a
--- /dev/null
+++ b/tools/perf/arch/riscv/util/tsc.c
@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/types.h>
+
+#include "../../../util/tsc.h"
+
+/*
+ * RV32 is left to the weak stub: reading the counter whole there takes time
+ * and timeh in a loop, and no RV32 user in tools/perf needs it that much.
+ */
+#if defined(__riscv) && __riscv_xlen == 64
+
+u64 rdtsc(void)
+{
+	u64 val;
+
+	/*
+	 * The time CSR ticks at a constant frequency and is the same counter
+	 * the kernel feeds into the user page conversion fields through
+	 * sched_clock(), so it is a TSC equivalent here.
+	 */
+	asm volatile("rdtime %0" : "=r" (val));
+
+	return val;
+}
+
+#endif
-- 
2.50.1

[PATCH v2 2/2] perf tsc: Declare TSC support from the build system

[Chen Pei]

The "TSC support" subtest of the "Convert perf time to TSC" test decides
whether the architecture is covered by matching compiler macros for
x86_64, i386 and arm64.  That list has to be edited for every new
architecture and can drift apart from the set of architectures that really
provide a counter.

Declare the capability where the implementation is, following the
PERF_HAVE_JITDUMP pattern: an architecture with a usable counter sets
PERF_HAVE_RDTSC in its arch Makefile, Makefile.config turns it into
-DHAVE_RDTSC, and the test keys off that.

x86 and arm64 keep the same behaviour, riscv starts running the subtest
where the previous patch defines rdtsc().

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
---
Changes in v2:
- Declare the capability through the build system instead of a weak
  arch__rdtsc_supported() hook.

 tools/perf/Makefile.config          |  4 ++++
 tools/perf/arch/arm64/Makefile      |  1 +
 tools/perf/arch/riscv/Makefile      |  1 +
 tools/perf/arch/x86/Makefile        |  1 +
 tools/perf/tests/perf-time-to-tsc.c | 19 ++++---------------
 5 files changed, 11 insertions(+), 15 deletions(-)

diff --git a/tools/perf/Makefile.config b/tools/perf/Makefile.config
index 4d5993da9f94..f1b3bf208231 100644
--- a/tools/perf/Makefile.config
+++ b/tools/perf/Makefile.config
@@ -618,6 +618,10 @@ ifdef PERF_HAVE_JITDUMP
   endif
 endif
 
+ifeq ($(PERF_HAVE_RDTSC),1)
+  CFLAGS += -DHAVE_RDTSC
+endif
+
 ifeq ($(SRCARCH),powerpc)
   ifndef NO_LIBDW
     CFLAGS += -DHAVE_SKIP_CALLCHAIN_IDX
diff --git a/tools/perf/arch/arm64/Makefile b/tools/perf/arch/arm64/Makefile
index 44cc3f023318..b9cd559065d6 100644
--- a/tools/perf/arch/arm64/Makefile
+++ b/tools/perf/arch/arm64/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 PERF_HAVE_JITDUMP := 1
+PERF_HAVE_RDTSC := 1
diff --git a/tools/perf/arch/riscv/Makefile b/tools/perf/arch/riscv/Makefile
index 44cc3f023318..8036bf55da8e 100644
--- a/tools/perf/arch/riscv/Makefile
+++ b/tools/perf/arch/riscv/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 PERF_HAVE_JITDUMP := 1
+PERF_HAVE_RDTSC := $(IS_64_BIT)
diff --git a/tools/perf/arch/x86/Makefile b/tools/perf/arch/x86/Makefile
index 44cc3f023318..b9cd559065d6 100644
--- a/tools/perf/arch/x86/Makefile
+++ b/tools/perf/arch/x86/Makefile
@@ -1,2 +1,3 @@
 # SPDX-License-Identifier: GPL-2.0
 PERF_HAVE_JITDUMP := 1
+PERF_HAVE_RDTSC := 1
diff --git a/tools/perf/tests/perf-time-to-tsc.c b/tools/perf/tests/perf-time-to-tsc.c
index f8f71fdd32b1..feb8063f8b59 100644
--- a/tools/perf/tests/perf-time-to-tsc.c
+++ b/tools/perf/tests/perf-time-to-tsc.c
@@ -22,16 +22,6 @@
 #include "tests.h"
 #include "util/sample.h"
 
-/*
- * Except x86_64/i386 and Arm64, other archs don't support TSC in perf.  Just
- * enable the test for x86_64/i386 and Arm64 archs.
- */
-#if defined(__x86_64__) || defined(__i386__) || defined(__aarch64__)
-#define TSC_IS_SUPPORTED 1
-#else
-#define TSC_IS_SUPPORTED 0
-#endif
-
 #define CHECK__(x) {				\
 	while ((x) < 0) {			\
 		pr_debug(#x " failed!\n");	\
@@ -49,11 +39,10 @@
 static int test__tsc_is_supported(struct test_suite *test __maybe_unused,
 				  int subtest __maybe_unused)
 {
-	if (!TSC_IS_SUPPORTED) {
-		pr_debug("Test not supported on this architecture\n");
-		return TEST_SKIP;
-	}
-
+#ifndef HAVE_RDTSC
+	pr_debug("Test not supported on this architecture\n");
+	return TEST_SKIP;
+#endif
 	return TEST_OK;
 }
 
-- 
2.50.1

Re: [PATCH v2 2/2] perf tsc: Declare TSC support from the build system

[Leo Yan]

On Tue, Sep 29, 2026 at 11:41:33AM +0800, Chen Pei wrote:

> x86 and arm64 keep the same behaviour, riscv starts running the subtest
> where the previous patch defines rdtsc().
> 
> Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>

For arm64:

Tested-by: Leo Yan <leo.yan@arm.com>