Re: [PATCH 2/2] perf tsc: Replace the compile-time TSC support check with a weak hook

[David Laight <david.laight.linux@gmail.com>]

On Mon, 28 Sep 2026 17:47:39 +0800
Chen Pei <cp0613@linux.alibaba.com> wrote:

> 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 it can drift apart from the set of architectures that
> really provide a counter: util/tsc.c already ships a weak rdtsc() which
> an architecture overrides with a strong definition when it has one.

If you mark the prototype in the .h file 'weak' you don't need a
default - instead check the symbol for being NULL.

Then you can do:
	if (!rdtsc)  {
 		pr_debug("Test not supported on this architecture\n");
 		return TEST_SKIP;
 	}

David

> 
> Add a weak arch__rdtsc_supported() next to the weak rdtsc(), let the
> test query it instead of the macros, and have x86, arm64 and riscv
> override it in the file where they implement rdtsc().  Support is then
> declared next to the counter it describes, and a caller can tell a stub
> apart from a real counter without keeping its own copy of the macro
> list.  This follows the pattern of the other optional per-architecture
> hooks in util/ such as get_cpuid_str().
> 
> x86 and arm64 keep the same behaviour, and riscv starts running the
> subtest because it gained rdtsc() in the previous patch.
> 
> Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
> ---
>  tools/perf/arch/arm64/util/tsc.c    |  5 +++++
>  tools/perf/arch/riscv/util/tsc.c    |  5 +++++
>  tools/perf/arch/x86/util/tsc.c      |  5 +++++
>  tools/perf/tests/perf-time-to-tsc.c | 12 +-----------
>  tools/perf/util/tsc.c               |  9 +++++++++
>  tools/perf/util/tsc.h               |  1 +
>  6 files changed, 26 insertions(+), 11 deletions(-)
> 
> diff --git a/tools/perf/arch/arm64/util/tsc.c b/tools/perf/arch/arm64/util/tsc.c
> index cc85bd9e73f1..fd7fba50d0a6 100644
> --- a/tools/perf/arch/arm64/util/tsc.c
> +++ b/tools/perf/arch/arm64/util/tsc.c
> @@ -19,3 +19,8 @@ u64 rdtsc(void)
>  
>  	return val;
>  }
> +
> +bool arch__rdtsc_supported(void)
> +{
> +	return true;
> +}
> diff --git a/tools/perf/arch/riscv/util/tsc.c b/tools/perf/arch/riscv/util/tsc.c
> index 7cc4710ac38c..312b0c755439 100644
> --- a/tools/perf/arch/riscv/util/tsc.c
> +++ b/tools/perf/arch/riscv/util/tsc.c
> @@ -17,3 +17,8 @@ u64 rdtsc(void)
>  
>  	return val;
>  }
> +
> +bool arch__rdtsc_supported(void)
> +{
> +	return true;
> +}
> diff --git a/tools/perf/arch/x86/util/tsc.c b/tools/perf/arch/x86/util/tsc.c
> index 3a439e4b12d2..6c01e96b5c2e 100644
> --- a/tools/perf/arch/x86/util/tsc.c
> +++ b/tools/perf/arch/x86/util/tsc.c
> @@ -17,6 +17,11 @@ u64 rdtsc(void)
>  	return low | ((u64)high) << 32;
>  }
>  
> +bool arch__rdtsc_supported(void)
> +{
> +	return true;
> +}
> +
>  /*
>   * Derive the TSC frequency in Hz from the /proc/cpuinfo, for example:
>   * ...
> diff --git a/tools/perf/tests/perf-time-to-tsc.c b/tools/perf/tests/perf-time-to-tsc.c
> index f8f71fdd32b1..c91e22d93b53 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");	\
> @@ -49,7 +39,7 @@
>  static int test__tsc_is_supported(struct test_suite *test __maybe_unused,
>  				  int subtest __maybe_unused)
>  {
> -	if (!TSC_IS_SUPPORTED) {
> +	if (!arch__rdtsc_supported()) {
>  		pr_debug("Test not supported on this architecture\n");
>  		return TEST_SKIP;
>  	}
> diff --git a/tools/perf/util/tsc.c b/tools/perf/util/tsc.c
> index ebf289bf6b9d..815621b1d112 100644
> --- a/tools/perf/util/tsc.c
> +++ b/tools/perf/util/tsc.c
> @@ -113,6 +113,15 @@ u64 __weak rdtsc(void)
>  	return 0;
>  }
>  
> +/*
> + * An architecture that implements rdtsc() with a real counter should override
> + * this, so that users of the counter can tell a stub apart from support.
> + */
> +bool __weak arch__rdtsc_supported(void)
> +{
> +	return false;
> +}
> +
>  size_t perf_event__fprintf_time_conv(union perf_event *event, FILE *fp)
>  {
>  	struct perf_record_time_conv *tc = (struct perf_record_time_conv *)event;
> diff --git a/tools/perf/util/tsc.h b/tools/perf/util/tsc.h
> index 57ce8449647f..83e76b207365 100644
> --- a/tools/perf/util/tsc.h
> +++ b/tools/perf/util/tsc.h
> @@ -25,6 +25,7 @@ int perf_read_tsc_conversion(const struct perf_event_mmap_page *pc,
>  u64 perf_time_to_tsc(u64 ns, struct perf_tsc_conversion *tc);
>  u64 tsc_to_perf_time(u64 cyc, struct perf_tsc_conversion *tc);
>  u64 rdtsc(void);
> +bool arch__rdtsc_supported(void);
>  u64 arch_get_tsc_freq(void);
>  
>  size_t perf_event__fprintf_time_conv(union perf_event *event, FILE *fp);