Patchwork [4/6] aarch64: Allow overriding HWCAP_CPUID feature check using HWCAP_MASK

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Submitter Siddhesh Poyarekar
Date June 1, 2017, 8:12 p.m.
Message ID <1496347928-19432-5-git-send-email-siddhesh@sourceware.org>
Download mbox | patch
Permalink /patch/20687/
State New
Headers show

Comments

Siddhesh Poyarekar - June 1, 2017, 8:12 p.m.
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
Adhemerval Zanella Netto - June 6, 2017, 6:24 p.m.
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.
Szabolcs Nagy - June 7, 2017, 3 p.m.
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)
Siddhesh Poyarekar - June 7, 2017, 4:36 p.m.
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

Patch

diff --git a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
index 7025062..ef6eecd 100644
--- a/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
+++ b/sysdeps/unix/sysv/linux/aarch64/cpu-features.c
@@ -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;
 }
diff --git a/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h b/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
new file mode 100644
index 0000000..7a60d72
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/aarch64/dl-procinfo.h
@@ -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 */