linux-kernel - [PATCH v2 2/6] selftests/x86: build fsgsbase

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Message-ID: <20240527210042.220315-3-jhubbard@nvidia.com>
Date: Mon, 27 May 2024 14:00:38 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Shuah Khan <shuah@...nel.org>
Cc: angquan yu <angquan21@...il.com>,
	"Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
	Ingo Molnar <mingo@...nel.org>,
	Binbin Wu <binbin.wu@...ux.intel.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Rick Edgecombe <rick.p.edgecombe@...el.com>,
	Sohil Mehta <sohil.mehta@...el.com>,
	Yu-cheng Yu <yu-cheng.yu@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Valentin Obst <kernel@...entinobst.de>,
	linux-kselftest@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>,
	llvm@...ts.linux.dev,
	x86@...nel.org,
	John Hubbard <jhubbard@...dia.com>
Subject: [PATCH v2 2/6] selftests/x86: build fsgsbase_restore.c with clang

First of all, in order to build with clang at all, one must first apply
Valentin Obst's build fix for LLVM [1]. Once that is done, then when
building with clang, via:

    make LLVM=1 -C tools/testing/selftests

Fix this by moving the inline asm to "pure" assembly, in two new files:
clang_helpers_32.S, clang_helpers_64.S.

As a bonus, the pure asm avoids the need for ifdefs, and is now very
simple and easy on the eyes.

[1] https://lore.kernel.org/all/20240329-selftests-libmk-llvm-rfc-v1-1-2f9ed7d1c49f@valentinobst.de/

Signed-off-by: John Hubbard <jhubbard@...dia.com>
---
 tools/testing/selftests/x86/Makefile           |  2 ++
 tools/testing/selftests/x86/clang_helpers_32.S | 11 +++++++++++
 tools/testing/selftests/x86/clang_helpers_64.S | 12 ++++++++++++
 tools/testing/selftests/x86/fsgsbase_restore.c | 11 +++++------
 4 files changed, 30 insertions(+), 6 deletions(-)
 create mode 100644 tools/testing/selftests/x86/clang_helpers_32.S
 create mode 100644 tools/testing/selftests/x86/clang_helpers_64.S

diff --git a/tools/testing/selftests/x86/Makefile b/tools/testing/selftests/x86/Makefile
index c1269466e0f8..99bc2ef84f5a 100644
--- a/tools/testing/selftests/x86/Makefile
+++ b/tools/testing/selftests/x86/Makefile
@@ -113,6 +113,8 @@ endef
 $(eval $(call extra-files,sysret_ss_attrs_64,thunks.S))
 $(eval $(call extra-files,ptrace_syscall_32,raw_syscall_helper_32.S))
 $(eval $(call extra-files,test_syscall_vdso_32,thunks_32.S))
+$(eval $(call extra-files,fsgsbase_restore_64,clang_helpers_64.S))
+$(eval $(call extra-files,fsgsbase_restore_32,clang_helpers_32.S))
 
 # check_initial_reg_state is special: it needs a custom entry, and it
 # needs to be static so that its interpreter doesn't destroy its initial
diff --git a/tools/testing/selftests/x86/clang_helpers_32.S b/tools/testing/selftests/x86/clang_helpers_32.S
new file mode 100644
index 000000000000..dc16271bac70
--- /dev/null
+++ b/tools/testing/selftests/x86/clang_helpers_32.S
@@ -0,0 +1,11 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * 32-bit assembly helpers for asm operations that lack support in both gcc and
+ * clang. For example, clang asm does not support segment prefixes.
+ */
+.global dereference_seg_base
+dereference_seg_base:
+	mov %fs:(0), %eax
+	ret
+
+.section .note.GNU-stack,"",%progbits
diff --git a/tools/testing/selftests/x86/clang_helpers_64.S b/tools/testing/selftests/x86/clang_helpers_64.S
new file mode 100644
index 000000000000..0d81c71cad97
--- /dev/null
+++ b/tools/testing/selftests/x86/clang_helpers_64.S
@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * 64-bit assembly helpers for asm operations that lack support in both gcc and
+ * clang. For example, clang asm does not support segment prefixes.
+ */
+.global dereference_seg_base
+
+dereference_seg_base:
+	mov %gs:(0), %rax
+	ret
+
+.section .note.GNU-stack,"",%progbits
diff --git a/tools/testing/selftests/x86/fsgsbase_restore.c b/tools/testing/selftests/x86/fsgsbase_restore.c
index 6fffadc51579..224058c1e4b2 100644
--- a/tools/testing/selftests/x86/fsgsbase_restore.c
+++ b/tools/testing/selftests/x86/fsgsbase_restore.c
@@ -39,12 +39,11 @@
 # define SEG "%fs"
 #endif
 
-static unsigned int dereference_seg_base(void)
-{
-	int ret;
-	asm volatile ("mov %" SEG ":(0), %0" : "=rm" (ret));
-	return ret;
-}
+/*
+ * Defined in clang_helpers_[32|64].S, because unlike gcc, clang inline asm does
+ * not support segmentation prefixes.
+ */
+unsigned int dereference_seg_base(void);
 
 static void init_seg(void)
 {
-- 
2.45.1