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AArch64: Check kernel version for SVE ifuncs

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Old Linux kernels disable SVE after every system call.  Calling the
SVE-optimized memcpy afterwards will then cause a trap to reenable SVE.
As a result, applications with a high use of syscalls may run slower with
the SVE memcpy.  This is true for kernels between 4.15.0 and before 6.2.0,
except for 5.14.0 which was patched.  Avoid this by checking the kernel
version and selecting the SVE ifunc on modern kernels.

Parse the kernel version reported by uname() into a 24-bit kernel.major.minor
value without calling any library functions.  If uname() is not supported or
if the version format is not recognized, assume the kernel is modern.

Tested-by: Florian Weimer <fweimer@redhat.com>
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
This commit is contained in:
Wilco Dijkstra
2024-03-21 16:48:33 +00:00
parent 1ea0511456
commit 2e94e2f5d2
5 changed files with 53 additions and 2 deletions
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48
sysdeps/unix/sysv/linux/aarch64/cpu-features.c
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@@ -21,6 +21,7 @@
#include <sys/auxv.h>
#include <elf/dl-hwcaps.h>
#include <sys/prctl.h>
#include <sys/utsname.h>
#include <dl-tunables-parse.h>
#define DCZID_DZP_MASK (1 << 4)
@@ -62,6 +63,46 @@ get_midr_from_mcpu (const struct tunable_str_t *mcpu)
return UINT64_MAX;
}
#if __LINUX_KERNEL_VERSION < 0x060200
/* Return true if we prefer using SVE in string ifuncs. Old kernels disable
SVE after every system call which results in unnecessary traps if memcpy
uses SVE. This is true for kernels between 4.15.0 and before 6.2.0, except
for 5.14.0 which was patched. For these versions return false to avoid using
SVE ifuncs.
Parse the kernel version into a 24-bit kernel.major.minor value without
calling any library functions. If uname() is not supported or if the version
format is not recognized, assume the kernel is modern and return true. */
static inline bool
prefer_sve_ifuncs (void)
{
struct utsname buf;
const char *p = &buf.release[0];
int kernel = 0;
int val;
if (__uname (&buf) < 0)
return true;
for (int shift = 16; shift >= 0; shift -= 8)
{
for (val = 0; *p >= '0' && *p <= '9'; p++)
val = val * 10 + *p - '0';
kernel |= (val & 255) << shift;
if (*p++ != '.')
break;
}
if (kernel >= 0x060200 || kernel == 0x050e00)
return true;
if (kernel >= 0x040f00)
return false;
return true;
}
#endif
static inline void
init_cpu_features (struct cpu_features *cpu_features)
{
@@ -126,6 +167,13 @@ init_cpu_features (struct cpu_features *cpu_features)
/* Check if SVE is supported. */
cpu_features->sve = GLRO (dl_hwcap) & HWCAP_SVE;
cpu_features->prefer_sve_ifuncs = cpu_features->sve;
#if __LINUX_KERNEL_VERSION < 0x060200
if (cpu_features->sve)
cpu_features->prefer_sve_ifuncs = prefer_sve_ifuncs ();
#endif
/* Check if MOPS is supported. */
cpu_features->mops = GLRO (dl_hwcap2) & HWCAP2_MOPS;
}