linux-user/aarch64: Add vdso

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

linux-user/aarch64/vdso.S

@@ -0,0 +1,71 @@
+/*
+ * aarch64 linux replacement vdso.
+ *
+ * Copyright 2023 Linaro, Ltd.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include <asm/unistd.h>
+
+/* ??? These are in include/elf.h, which is not ready for inclusion in asm. */
+#define NT_GNU_PROPERTY_TYPE_0  5
+#define GNU_PROPERTY_AARCH64_FEATURE_1_AND      0xc0000000
+#define GNU_PROPERTY_AARCH64_FEATURE_1_BTI      (1U << 0)
+#define GNU_PROPERTY_AARCH64_FEATURE_1_PAC      (1U << 1)
+
+#define GNU_PROPERTY_AARCH64_FEATURE_1_DEFAULT \
+    (GNU_PROPERTY_AARCH64_FEATURE_1_BTI | GNU_PROPERTY_AARCH64_FEATURE_1_PAC)
+
+	.section .note.gnu.property
+	.align	3
+	.long	2f - 1f
+	.long	6f - 3f
+	.long	NT_GNU_PROPERTY_TYPE_0
+1:	.string	"GNU"
+2:	.align	3
+3:	.long	GNU_PROPERTY_AARCH64_FEATURE_1_AND
+	.long	5f - 4f
+4:	.long	GNU_PROPERTY_AARCH64_FEATURE_1_DEFAULT
+5:	.align	3
+6:
+
+	.text
+
+.macro endf name
+	.globl	\name
+	.type	\name, @function
+	.size	\name, . - \name
+.endm
+
+.macro vdso_syscall name, nr
+\name:
+	bti	c
+	mov	x8, #\nr
+	svc	#0
+	ret
+endf	\name
+.endm
+
+	.cfi_startproc
+
+vdso_syscall __kernel_gettimeofday, __NR_gettimeofday
+vdso_syscall __kernel_clock_gettime, __NR_clock_gettime
+vdso_syscall __kernel_clock_getres, __NR_clock_getres
+
+	.cfi_endproc
+
+
+/*
+ * TODO: The kernel makes a big deal of turning off the .cfi directives,
+ * because they cause libgcc to crash, but that's because they're wrong.
+ *
+ * For now, elide the unwind info for __kernel_rt_sigreturn and rely on
+ * the libgcc fallback routine as we have always done.  This requires
+ * that the code sequence used be exact.
+ */
+__kernel_rt_sigreturn:
+	/* No BTI C insn here -- we arrive via RET. */
+	mov	x8, #__NR_rt_sigreturn
+	svc	#0
+endf	__kernel_rt_sigreturn