x86: Inline TSC reads in timekeeping

Avoid the overhead of the indirect call for a single instruction to read
the TSC.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260224163429.741886362@kernel.org

arch/x86/Kconfig

@@ -141,6 +141,7 @@ config X86
 	select ARCH_USE_SYM_ANNOTATIONS
 	select ARCH_WANT_BATCHED_UNMAP_TLB_FLUSH
 	select ARCH_WANT_DEFAULT_BPF_JIT	if X86_64
+	select ARCH_WANTS_CLOCKSOURCE_READ_INLINE	if X86_64
 	select ARCH_WANTS_DYNAMIC_TASK_STRUCT
 	select ARCH_WANTS_NO_INSTR
 	select ARCH_WANT_GENERAL_HUGETLB

arch/x86/include/asm/clock_inlined.h

@@ -0,0 +1,14 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_CLOCK_INLINED_H
+#define _ASM_X86_CLOCK_INLINED_H
+
+#include <asm/tsc.h>
+
+struct clocksource;
+
+static __always_inline u64 arch_inlined_clocksource_read(struct clocksource *cs)
+{
+	return (u64)rdtsc_ordered();
+}
+
+#endif

arch/x86/kernel/tsc.c

@@ -1201,6 +1201,7 @@ static struct clocksource clocksource_tsc = {
 	.mask			= CLOCKSOURCE_MASK(64),
 	.flags			= CLOCK_SOURCE_IS_CONTINUOUS |
 				  CLOCK_SOURCE_VALID_FOR_HRES |
+				  CLOCK_SOURCE_CAN_INLINE_READ |
 				  CLOCK_SOURCE_MUST_VERIFY |
 				  CLOCK_SOURCE_VERIFY_PERCPU,
 	.id			= CSID_X86_TSC,