[4/6] aarch64: Allow overriding HWCAP_CPUID feature check using HWCAP_MASK
Commit Message
Now that LD_HWCAP_MASK (or glibc.tune.hwcap_mask) is read early enough
to influence cpu feature check in aarch64, use it to influence
multiarch selection. Setting LD_HWCAP_MASK such that it clears
HWCAP_CPUID will now disable multiarch for the binary.
HWCAP_CPUID is also now set in HWCAP_IMPORTANT so that it is set by
default. With this patch, this feature is only usable with
dyanmically linked binaries because LD_HWCAP_MASK is not read for
static binaries. A future patch fixes that.
* sysdeps/unix/sysv/linux/aarch64/cpu-features.c
(init_cpu_features): Use glibc.tune.hwcap_mask.
* sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h: New file.
---
sysdeps/unix/sysv/linux/aarch64/cpu-features.c | 10 +++---
sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h | 44 ++++++++++++++++++++++++++
2 files changed, 50 insertions(+), 4 deletions(-)
create mode 100644 sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
Comments
On 01/06/2017 17:12, Siddhesh Poyarekar wrote:
> Now that LD_HWCAP_MASK (or glibc.tune.hwcap_mask) is read early enough
> to influence cpu feature check in aarch64, use it to influence
> multiarch selection. Setting LD_HWCAP_MASK such that it clears
> HWCAP_CPUID will now disable multiarch for the binary.
>
> HWCAP_CPUID is also now set in HWCAP_IMPORTANT so that it is set by
> default. With this patch, this feature is only usable with
> dyanmically linked binaries because LD_HWCAP_MASK is not read for
> static binaries. A future patch fixes that.
>
> * sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> (init_cpu_features): Use glibc.tune.hwcap_mask.
> * sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h: New file.
LGTM.
On 01/06/17 21:12, Siddhesh Poyarekar wrote:
> Now that LD_HWCAP_MASK (or glibc.tune.hwcap_mask) is read early enough
> to influence cpu feature check in aarch64, use it to influence
> multiarch selection. Setting LD_HWCAP_MASK such that it clears
> HWCAP_CPUID will now disable multiarch for the binary.
>
> HWCAP_CPUID is also now set in HWCAP_IMPORTANT so that it is set by
> default. With this patch, this feature is only usable with
> dyanmically linked binaries because LD_HWCAP_MASK is not read for
> static binaries. A future patch fixes that.
>
> * sysdeps/unix/sysv/linux/aarch64/cpu-features.c
> (init_cpu_features): Use glibc.tune.hwcap_mask.
> * sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h: New file.
> ---
> sysdeps/unix/sysv/linux/aarch64/cpu-features.c | 10 +++---
> sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h | 44 ++++++++++++++++++++++++++
> 2 files changed, 50 insertions(+), 4 deletions(-)
> create mode 100644 sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
>
OK if the new HWCAP_IMPORTANT does not cause
additional open/stat/.. syscalls (please verify
that the lib search path is not increased by
running something like
strace -E LD_PRELOAD=foobar.so /bin/true
and counting the failed syscalls)
(if the search path is increased then i'd like
to know what is the purpose of that feature)
On Wednesday 07 June 2017 08:30 PM, Szabolcs Nagy wrote:
> OK if the new HWCAP_IMPORTANT does not cause
> additional open/stat/.. syscalls (please verify
> that the lib search path is not increased by
> running something like
> strace -E LD_PRELOAD=foobar.so /bin/true
> and counting the failed syscalls)
>
> (if the search path is increased then i'd like
> to know what is the purpose of that feature)
I thought I already explained it in a previous email - this adds an
additional search path for the dynamic linker to look for libraries that
may be optimized for that specific hwcap bit. A straightforward way to
disable this is to set LD_HWCAP_MASK to 0x0 to clear hwcap bits.
On server systems where this would be used, this would have been done by
ldconfig and hence the lookup will not involve a full search every time,
just for situations where the library is not mapped into ld.so.cache.
Siddhesh
@@ -18,18 +18,20 @@
#include <cpu-features.h>
#include <sys/auxv.h>
+#include <elf/dl-hwcaps.h>
static inline void
init_cpu_features (struct cpu_features *cpu_features)
{
- if (GLRO(dl_hwcap) & HWCAP_CPUID)
+ uint64_t hwcap_mask = GET_HWCAP_MASK();
+ uint64_t hwcap = GLRO (dl_hwcap) & hwcap_mask;
+
+ if (hwcap & HWCAP_CPUID)
{
register uint64_t id = 0;
asm volatile ("mrs %0, midr_el1" : "=r"(id));
cpu_features->midr_el1 = id;
}
else
- {
- cpu_features->midr_el1 = 0;
- }
+ cpu_features->midr_el1 = 0;
}
new file mode 100644
@@ -0,0 +1,44 @@
+/* Processor capability information handling macros - aarch64 version.
+ Copyright (C) 2017 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <http://www.gnu.org/licenses/>. */
+
+#ifndef _DL_PROCINFO_H
+#define _DL_PROCINFO_H 1
+
+#include <sys/auxv.h>
+
+/* We cannot provide a general printing function. */
+#define _dl_procinfo(type, word) -1
+
+/* There are no hardware capabilities defined. */
+#define _dl_hwcap_string(idx) ""
+
+/* HWCAP_CPUID should be available by default to influence IFUNC as well as
+ library search. */
+#define HWCAP_IMPORTANT HWCAP_CPUID
+
+/* There're no platforms to filter out. */
+#define _DL_HWCAP_PLATFORM 0
+
+/* We don't have any hardware capabilities. */
+#define _DL_HWCAP_COUNT 0
+
+#define _dl_string_hwcap(str) (-1)
+
+#define _dl_string_platform(str) (-1)
+
+#endif /* dl-procinfo.h */