linux-user/x86_64: Add vdso

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

linux-user/x86_64/vdso.S

@@ -0,0 +1,78 @@
+/*
+ * x86-64 linux replacement vdso.
+ *
+ * Copyright 2023 Linaro, Ltd.
+ *
+ * SPDX-License-Identifier: GPL-2.0-or-later
+ */
+
+#include <asm/unistd.h>
+
+.macro endf name
+	.globl	\name
+	.type	\name, @function
+	.size	\name, . - \name
+.endm
+
+.macro weakalias name
+\name	= __vdso_\name
+	.weak	\name
+.endm
+
+.macro vdso_syscall name, nr
+__vdso_\name:
+	mov	$\nr, %eax
+	syscall
+	ret
+endf	__vdso_\name
+weakalias \name
+.endm
+
+	.cfi_startproc
+
+vdso_syscall clock_gettime, __NR_clock_gettime
+vdso_syscall clock_getres, __NR_clock_getres
+vdso_syscall gettimeofday, __NR_gettimeofday
+vdso_syscall time, __NR_time
+
+__vdso_getcpu:
+	/*
+         * There is no syscall number for this allocated on x64.
+	 * We can handle this several ways:
+         *
+	 * (1) Invent a syscall number for use within qemu.
+         *     It should be easy enough to pick a number that
+         *     is well out of the way of the kernel numbers.
+         *
+         * (2) Force the emulated cpu to support the rdtscp insn,
+	 *     and initialize the TSC_AUX value the appropriate value.
+         *
+	 * (3) Pretend that we're always running on cpu 0.
+         *
+	 * This last is the one that's implemented here, with the
+	 * tiny bit of extra code to support rdtscp in place.
+         */
+	xor	%ecx, %ecx		/* rdtscp w/ tsc_aux = 0 */
+
+	/* if (cpu != NULL) *cpu = (ecx & 0xfff); */
+	test	%rdi, %rdi
+	jz	1f
+	mov	%ecx, %eax
+	and	$0xfff, %eax
+	mov	%eax, (%rdi)
+
+	/* if (node != NULL) *node = (ecx >> 12); */
+1:	test	%rsi, %rsi
+	jz	2f
+	shr	$12, %ecx
+	mov	%ecx, (%rsi)
+
+2:	xor	%eax, %eax
+	ret
+endf	__vdso_getcpu
+
+weakalias getcpu
+
+	.cfi_endproc
+
+/* TODO: Add elf note for LINUX_VERSION_CODE */