[PATCH] x86: Set FSGSBASE to active if enabled by kernel

[PATCH] x86: Set FSGSBASE to active if enabled by kernel

[H.J. Lu]

Linux kernel uses AT_HWCAP2 to indicate if FSGSBASE instructions are
enabled.  If the HWCAP2_FSGSBASE bit in AT_HWCAP2 is set, FSGSBASE
instructions can be used in user space.  Define dl_check_hwcap2 to set
the FSGSBASE feature to active on Linux when the HWCAP2_FSGSBASE bit is
set.
---
 sysdeps/unix/sysv/linux/x86/dl-hwcap2.h | 27 +++++++++++++++++++++++++
 sysdeps/x86/cpu-features.c              |  3 +++
 sysdeps/x86/dl-hwcap2.h                 | 21 +++++++++++++++++++
 3 files changed, 51 insertions(+)
 create mode 100644 sysdeps/unix/sysv/linux/x86/dl-hwcap2.h
 create mode 100644 sysdeps/x86/dl-hwcap2.h

diff --git a/sysdeps/unix/sysv/linux/x86/dl-hwcap2.h b/sysdeps/unix/sysv/linux/x86/dl-hwcap2.h
new file mode 100644
index 0000000000..28f38ea510
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/x86/dl-hwcap2.h
@@ -0,0 +1,27 @@
+/* Handle AT_HWCAP2.  Linux/x86 version.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+
+   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
+   <https://www.gnu.org/licenses/>.  */
+
+/* The following must match the kernel's <asm/hwcap2.h>.  */
+#define HWCAP2_RING3MWAIT	(1 << 0)
+#define HWCAP2_FSGSBASE		(1 << 1)
+
+static inline void
+dl_check_hwcap2 (struct cpu_features *cpu_features)
+{
+  if ((GLRO(dl_hwcap2) & HWCAP2_FSGSBASE) != 0)
+    CPU_FEATURE_SET_ACTIVE (cpu_features, FSGSBASE);
+}
diff --git a/sysdeps/x86/cpu-features.c b/sysdeps/x86/cpu-features.c
index 822688e21f..05f966ea64 100644
--- a/sysdeps/x86/cpu-features.c
+++ b/sysdeps/x86/cpu-features.c
@@ -22,6 +22,7 @@
 #include <cacheinfo.h>
 #include <dl-cacheinfo.h>
 #include <dl-minsigstacksize.h>
+#include <dl-hwcap2.h>
 
 #if HAVE_TUNABLES
 extern void TUNABLE_CALLBACK (set_hwcaps) (tunable_val_t *)
@@ -303,6 +304,8 @@ update_active (struct cpu_features *cpu_features)
       CPU_FEATURE_SET_ACTIVE (cpu_features, WIDE_KL);
     }
 
+  dl_check_hwcap2 (cpu_features);
+
   cpu_features->isa_1 = get_isa_level (cpu_features);
 }
 
diff --git a/sysdeps/x86/dl-hwcap2.h b/sysdeps/x86/dl-hwcap2.h
new file mode 100644
index 0000000000..73cd6cc37d
--- /dev/null
+++ b/sysdeps/x86/dl-hwcap2.h
@@ -0,0 +1,21 @@
+/* Handle AT_HWCAP2.  Generic x86 version.
+   Copyright (C) 2023 Free Software Foundation, Inc.
+
+   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
+   <https://www.gnu.org/licenses/>.  */
+
+static inline void
+dl_check_hwcap2 (struct cpu_features *cpu_features)
+{
+}
-- 
2.39.2

Re: [PATCH] x86: Set FSGSBASE to active if enabled by kernel

[Florian Weimer]

* H. J. Lu:

> Linux kernel uses AT_HWCAP2 to indicate if FSGSBASE instructions are
> enabled.  If the HWCAP2_FSGSBASE bit in AT_HWCAP2 is set, FSGSBASE
> instructions can be used in user space.  Define dl_check_hwcap2 to set
> the FSGSBASE feature to active on Linux when the HWCAP2_FSGSBASE bit is
> set.

Could you add a test against /proc/cpuinfo flags?  It should be a
perfect match on current kernels.

Thanks,
Florian

Re: [PATCH] x86: Set FSGSBASE to active if enabled by kernel

[H.J. Lu]

On Mon, Mar 13, 2023 at 6:44 AM Florian Weimer <fweimer@redhat.com> wrote:
>
> * H. J. Lu:
>
> > Linux kernel uses AT_HWCAP2 to indicate if FSGSBASE instructions are
> > enabled.  If the HWCAP2_FSGSBASE bit in AT_HWCAP2 is set, FSGSBASE
> > instructions can be used in user space.  Define dl_check_hwcap2 to set
> > the FSGSBASE feature to active on Linux when the HWCAP2_FSGSBASE bit is
> > set.
>
> Could you add a test against /proc/cpuinfo flags?  It should be a
> perfect match on current kernels.
>

Will do.

Thanks.

-- 
H.J.