Re: [PATCH v4 1/2] perf env: Add perf_env__e_machine helper and use in perf_env__arch

Ian Rogers
2026-04-06T15:36:21Z

On Sun, Apr 5, 2026 at 10:05 PM Namhyung Kim <namhyung@kernel.org> wrote:
>
> On Thu, Mar 26, 2026 at 09:50:24PM -0700, Ian Rogers wrote:
> > Add a helper that lazily computes the e_machine and falls back of
> > EM_HOST. Use the perf_env's arch to compute the e_machine if
> > available. Use a binary search for some efficiency in this, but handle
> > somewhat complex duplicate rules. Switch perf_env__arch to be derived
> > the e_machine for consistency. This switches arch from being uname
> > derived to matching that of the perf binary (via EM_HOST). Update
> > session to use the helper, which may mean using EM_HOST when no
> > threads are available. This also updates the perf data file header
> > that gets the e_machine/e_flags from the session.
> >
> > Signed-off-by: Ian Rogers <irogers@google.com>
> > ---
> > tools/perf/util/env.c | 185 ++++++++++++++++++++++++++++++--------
> > tools/perf/util/env.h | 1 +
> > tools/perf/util/session.c | 14 +--
> > 3 files changed, 157 insertions(+), 43 deletions(-)
> >
> > diff --git a/tools/perf/util/env.c b/tools/perf/util/env.c
> > index 93d475a80f14..ae08178870d7 100644
> > --- a/tools/perf/util/env.c
> > +++ b/tools/perf/util/env.c
> > @@ -1,10 +1,12 @@
> > // SPDX-License-Identifier: GPL-2.0
> > #include "cpumap.h"
> > +#include "dwarf-regs.h"
> > #include "debug.h"
> > #include "env.h"
> > #include "util/header.h"
> > #include "util/rwsem.h"
> > #include <linux/compiler.h>
> > +#include <linux/kernel.h>
> > #include <linux/ctype.h>
> > #include <linux/rbtree.h>
> > #include <linux/string.h>
> > @@ -588,51 +590,160 @@ void cpu_cache_level__free(struct cpu_cache_level *cache)
> > zfree(&cache->size);
> > }
> >
> > +struct arch_to_e_machine {
> > + const char *prefix;
> > + uint16_t e_machine;
> > +};
> > +
> > /*
> > - * Return architecture name in a normalized form.
> > - * The conversion logic comes from the Makefile.
> > + * A mapping from an arch prefix string to an ELF machine that can be used in a
> > + * bsearch. Some arch prefixes are shared an need additional processing as
> > + * marked next to the architecture. The prefixes handle both perf's architecture
> > + * naming and those from uname.
> > */
> > -static const char *normalize_arch(char *arch)
> > -{
> > - if (!strcmp(arch, "x86_64"))
> > - return "x86";
> > - if (arch[0] == 'i' && arch[2] == '8' && arch[3] == '6')
> > - return "x86";
> > - if (!strcmp(arch, "sun4u") || !strncmp(arch, "sparc", 5))
> > - return "sparc";
> > - if (!strncmp(arch, "aarch64", 7) || !strncmp(arch, "arm64", 5))
> > - return "arm64";
> > - if (!strncmp(arch, "arm", 3) || !strcmp(arch, "sa110"))
> > - return "arm";
> > - if (!strncmp(arch, "s390", 4))
> > - return "s390";
> > - if (!strncmp(arch, "parisc", 6))
> > - return "parisc";
> > - if (!strncmp(arch, "powerpc", 7) || !strncmp(arch, "ppc", 3))
> > - return "powerpc";
> > - if (!strncmp(arch, "mips", 4))
> > - return "mips";
> > - if (!strncmp(arch, "sh", 2) && isdigit(arch[2]))
> > - return "sh";
> > - if (!strncmp(arch, "loongarch", 9))
> > - return "loongarch";
> > -
> > - return arch;
> > +static const struct arch_to_e_machine prefix_to_e_machine[] = {
> > + {"aarch64", EM_AARCH64},
> > + {"alpha", EM_ALPHA},
> > + {"arc", EM_ARC},
> > + {"arm", EM_ARM}, /* Check also for EM_AARCH64. */
> > + {"avr", EM_AVR}, /* Check also for EM_AVR32. */
> > + {"bfin", EM_BLACKFIN},
> > + {"blackfin", EM_BLACKFIN},
> > + {"cris", EM_CRIS},
> > + {"csky", EM_CSKY},
> > + {"hppa", EM_PARISC},
> > + {"i386", EM_386},
> > + {"i486", EM_386},
> > + {"i586", EM_386},
> > + {"i686", EM_386},
> > + {"loongarch", EM_LOONGARCH},
> > + {"m32r", EM_M32R},
> > + {"m68k", EM_68K},
> > + {"microblaze", EM_MICROBLAZE},
> > + {"mips", EM_MIPS},
> > + {"msp430", EM_MSP430},
> > + {"parisc", EM_PARISC},
> > + {"powerpc", EM_PPC}, /* Check also for EM_PPC64. */
> > + {"ppc", EM_PPC}, /* Check also for EM_PPC64. */
> > + {"riscv", EM_RISCV},
> > + {"s390", EM_S390},
> > + {"sa110", EM_ARM},
> > + {"sh", EM_SH},
> > + {"sparc", EM_SPARC}, /* Check also for EM_SPARCV9. */
> > + {"sun4u", EM_SPARC},
> > + {"x86", EM_X86_64}, /* Check also for EM_386. */
> > + {"xtensa", EM_XTENSA},
> > +};
> > +
> > +static int compare_prefix(const void *key, const void *element)
> > +{
> > + const char *search_key = key;
> > + const struct arch_to_e_machine *map_element = element;
> > + size_t prefix_len = strlen(map_element->prefix);
> > +
> > + return strncmp(search_key, map_element->prefix, prefix_len);
> > +}
> > +
> > +static uint16_t perf_arch_to_e_machine(const char *perf_arch, bool is_64_bit)
> > +{
> > + /* Binary search for a matching prefix. */
> > + const struct arch_to_e_machine *result;
> > +
> > + if (!perf_arch)
> > + return EM_HOST;
> > +
> > + result = bsearch(perf_arch,
> > + prefix_to_e_machine, ARRAY_SIZE(prefix_to_e_machine),
> > + sizeof(prefix_to_e_machine[0]),
> > + compare_prefix);
> > +
> > + if (!result) {
> > + pr_debug("Unknown perf arch for ELF machine mapping: %s\n", perf_arch);
> > + return EM_NONE;
> > + }
> > +
> > + /* Handle conflicting prefixes. */
> > + switch (result->e_machine) {
> > + case EM_ARM:
> > + return !strcmp(perf_arch, "arm64") ? EM_AARCH64 : EM_ARM;
> > + case EM_AVR:
> > + return !strcmp(perf_arch, "avr32") ? EM_AVR32 : EM_AVR;
> > + case EM_PPC:
> > + return is_64_bit || strstarts(perf_arch, "ppc64") ? EM_PPC64 : EM_PPC;
>
> I'm curious what's the name `uname -m` returns for PPC64. Is
> "powerpc64" possible?

It is.

> > + case EM_SPARC:
> > + return is_64_bit || !strcmp(perf_arch, "sparc64") ? EM_SPARCV9 : EM_SPARC;
> > + case EM_X86_64:
> > + return is_64_bit || !strcmp(perf_arch, "x86_64") ? EM_X86_64 : EM_386;
> > + default:
> > + return result->e_machine;
> > + }
> > +}
> > +
> > +static const char *e_machine_to_perf_arch(uint16_t e_machine)
> > +{
> > + /*
> > + * Table for if either the perf arch string differs from uname or there
> > + * are >1 ELF machine with the prefix.
> > + */
> > + static const struct arch_to_e_machine extras[] = {
> > + {"arm64", EM_AARCH64},
> > + {"avr32", EM_AVR32},
> > + {"powerpc", EM_PPC},
> > + {"powerpc", EM_PPC64},
>
> Here it returns powerpc for both.

Yep. This is 100% intentional as the existing code does the same:
https://web.git.kernel.org/pub/scm/linux/kernel/git/perf/perf-tools-next.git/tree/tools/perf/util/env.c?h=perf-tools-next#n611
```
static const char *normalize_arch(char *arch)
...
if (!strncmp(arch, "powerpc", 7) || !strncmp(arch, "ppc", 3))
return "powerpc";
```
The strncmp is limited to just the prefix of the uname string,
ignoring the 64. So the arch "powerpc" can be 32-bit or 64-bit, just
as "x86" can be 32-bit or 64-bit. To determine which case applies, the
code should really check `struct perf_env`'s `kernel_is_64_bit`. I
think this is generally much more painful than just using the
e_machine - especially since you need to strcmp the name. For the
e_machine, the problem is that on x86 we have 32-bit, x32 and x86_64.
There is then also an ABI question regarding the use of SIMD registers
and the newer APX registers. If there are no samples and no DSOs in
play, making a choice of e_machine to set up variables with is
somewhat arbitrary. I think EM_HOST, the e_machine of the current perf
binary, is a good choice.

Thanks,
Ian

> > + {"sparc", EM_SPARCV9},
> > + {"x86", EM_386},
> > + {"x86", EM_X86_64},
> > + {"none", EM_NONE},
> > + };
> > +
> > + for (size_t i = 0; i < ARRAY_SIZE(extras); i++) {
> > + if (extras[i].e_machine == e_machine)
> > + return extras[i].prefix;
> > + }
> > +
> > + for (size_t i = 0; i < ARRAY_SIZE(prefix_to_e_machine); i++) {
> > + if (prefix_to_e_machine[i].e_machine == e_machine)
> > + return prefix_to_e_machine[i].prefix;
> > +
> > + }
> > + return "unknown";
> > +}
> > +
> > +uint16_t perf_env__e_machine(struct perf_env *env, uint32_t *e_flags)
> > +{
> > + if (!env) {
> > + if (e_flags)
> > + *e_flags = EF_HOST;
> > +
> > + return EM_HOST;
> > + }
> > + if (env->e_machine == EM_NONE) {
> > + env->e_machine = perf_arch_to_e_machine(env->arch, env->kernel_is_64_bit);
> > +
> > + if (env->e_machine == EM_HOST)
> > + env->e_flags = EF_HOST;
> > + }
> > + if (e_flags)
> > + *e_flags = env->e_flags;
> > +
> > + return env->e_machine;
> > }
> >
> > const char *perf_env__arch(struct perf_env *env)
> > {
> > - char *arch_name;
> > + if (!env)
> > + return e_machine_to_perf_arch(EM_HOST);
> >
> > - if (!env || !env->arch) { /* Assume local operation */
> > - static struct utsname uts = { .machine[0] = '\0', };
> > - if (uts.machine[0] == '\0' && uname(&uts) < 0)
> > - return NULL;
> > - arch_name = uts.machine;
> > - } else
> > - arch_name = env->arch;
> > + if (!env->arch) {
> > + /*
> > + * Lazily compute/allocate arch. The e_machine may have been
> > + * read from a data file and so may not be EM_HOST.
> > + */
> > + uint16_t e_machine = perf_env__e_machine(env, /*e_flags=*/NULL);
> >
> > - return normalize_arch(arch_name);
> > + env->arch = strdup(e_machine_to_perf_arch(e_machine));
> > + }
> > + return env->arch;
> > }
> >
> > #if defined(HAVE_LIBTRACEEVENT)
> > diff --git a/tools/perf/util/env.h b/tools/perf/util/env.h
> > index a4501cbca375..91ff252712f4 100644
> > --- a/tools/perf/util/env.h
> > +++ b/tools/perf/util/env.h
> > @@ -186,6 +186,7 @@ int perf_env__read_cpu_topology_map(struct perf_env *env);
> >
> > void cpu_cache_level__free(struct cpu_cache_level *cache);
> >
> > +uint16_t perf_env__e_machine(struct perf_env *env, uint32_t *e_flags);
> > const char *perf_env__arch(struct perf_env *env);
> > const char *perf_env__arch_strerrno(struct perf_env *env, int err);
> > const char *perf_env__cpuid(struct perf_env *env);
> > diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
> > index 4b465abfa36c..dcc9bef303aa 100644
> > --- a/tools/perf/util/session.c
> > +++ b/tools/perf/util/session.c
> > @@ -2996,14 +2996,16 @@ uint16_t perf_session__e_machine(struct perf_session *session, uint32_t *e_flags
> > return EM_HOST;
> > }
> >
> > + /* Is the env caching an e_machine? */
> > env = perf_session__env(session);
> > - if (env && env->e_machine != EM_NONE) {
> > - if (e_flags)
> > - *e_flags = env->e_flags;
> > -
> > - return env->e_machine;
> > - }
> > + if (env && env->e_machine != EM_NONE)
> > + return perf_env__e_machine(env, e_flags);
> >
> > + /*
> > + * Compute from threads, note this is more accurate than
> > + * perf_env__e_machine that falls back on EM_HOST and doesn't consider
> > + * mixed 32-bit and 64-bit threads.
> > + */
> > machines__for_each_thread(&session->machines,
> > perf_session__e_machine_cb,
> > &args);
> > --
> > 2.53.0.1018.g2bb0e51243-goog
> >