[RFC PATCH 19/46] x86: liveupdate: Add low-level physical CPU preservation assembly

[Pasha Tatashin]

Add arch/x86/kernel/preserve_cpu.S stack switch routine, preserved linker
sections in vmlinux.lds.S, and SMP stop exclusion for preserved cores.

Signed-off-by: Pasha Tatashin <pasha.tatashin at soleen.com>
---
 arch/x86/include/asm/cpu_preserve.h | 31 +++++++++++++++
 arch/x86/kernel/preserve_cpu.S      | 62 +++++++++++++++++++++++++++++
 arch/x86/kernel/vmlinux.lds.S       |  2 +
 3 files changed, 95 insertions(+)
 create mode 100644 arch/x86/include/asm/cpu_preserve.h
 create mode 100644 arch/x86/kernel/preserve_cpu.S

diff --git a/arch/x86/include/asm/cpu_preserve.h b/arch/x86/include/asm/cpu_preserve.h
new file mode 100644
index 000000000000..969903eb70cb
--- /dev/null
+++ b/arch/x86/include/asm/cpu_preserve.h
@@ -0,0 +1,31 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2026, Google LLC.
+ * Pasha Tatashin <pasha.tatashin at soleen.com>
+ */
+#ifndef __ASM_X86_CPU_PRESERVE_H
+#define __ASM_X86_CPU_PRESERVE_H
+
+#include <asm/page_types.h>
+
+#define ARCH_CPU_PRESERVED_STACK_ORDER	THREAD_SIZE_ORDER
+
+#ifdef CONFIG_CC_IS_GCC
+#define ARCH_CPU_PRESERVED_TEXT \
+	__attribute__((indirect_branch("keep"), function_return("keep")))
+#else
+#define ARCH_CPU_PRESERVED_TEXT
+#endif
+
+#ifdef CONFIG_LIVEUPDATE_CPU
+void arch_cpu_preserved_load_desc(void);
+bool arch_cpu_preserved_is_active(void);
+void x86_preserved_iret_stub(void);
+void x86_preserved_iret_err_stub(void);
+void x86_preserved_apic_eoi_stub(void);
+#else
+static inline void arch_cpu_preserved_load_desc(void) {}
+static inline bool arch_cpu_preserved_is_active(void) { return false; }
+#endif
+
+#endif /* __ASM_X86_CPU_PRESERVE_H */
diff --git a/arch/x86/kernel/preserve_cpu.S b/arch/x86/kernel/preserve_cpu.S
new file mode 100644
index 000000000000..8acb98d46f90
--- /dev/null
+++ b/arch/x86/kernel/preserve_cpu.S
@@ -0,0 +1,62 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+/*
+ * Copyright (c) 2026, Google LLC.
+ * Pasha Tatashin <pasha.tatashin at soleen.com>
+ */
+#include <linux/linkage.h>
+#include <asm/nospec-branch.h>
+#include <asm/unwind_hints.h>
+
+	.section .text.cpu_preserved, "ax"
+
+SYM_CODE_START(x86_preserved_iret_err_stub)
+	UNWIND_HINT_IRET_REGS
+	ENDBR
+	addq	$8, %rsp
+	iretq
+SYM_CODE_END(x86_preserved_iret_err_stub)
+
+SYM_CODE_START(x86_preserved_iret_stub)
+	UNWIND_HINT_IRET_REGS
+	ENDBR
+	iretq
+SYM_CODE_END(x86_preserved_iret_stub)
+
+SYM_CODE_START(x86_preserved_apic_eoi_stub)
+	UNWIND_HINT_IRET_REGS
+	ENDBR
+	pushq	%rax
+	pushq	%rcx
+	pushq	%rdx
+	/* x2APIC mode: MSR 0x80b (APIC_EOI) = 0 */
+	movl	$0x80b, %ecx
+	xorl	%eax, %eax
+	xorl	%edx, %edx
+	wrmsr
+	popq	%rdx
+	popq	%rcx
+	popq	%rax
+	iretq
+SYM_CODE_END(x86_preserved_apic_eoi_stub)
+
+/*
+ * arch_cpu_preserved_call_on_stack - Switch stack and call function
+ * @cpu: CPU ID (%rdi)
+ * @stack: Target stack top (%rsi)
+ * @fn: Function pointer to invoke (%rdx)
+ */
+SYM_CODE_START(arch_cpu_preserved_call_on_stack)
+	pushq	%rbp
+	movq	%rsp, %rbp
+	pushq	%r12
+
+	movq	%rsp, %r12	/* save old stack pointer */
+	movq	%rsi, %rsp	/* switch to target stack */
+
+	call	*%rdx		/* call fn(cpu) */
+
+	movq	%r12, %rsp	/* restore old stack pointer */
+	popq	%r12
+	leave
+	ret
+SYM_CODE_END(arch_cpu_preserved_call_on_stack)
diff --git a/arch/x86/kernel/vmlinux.lds.S b/arch/x86/kernel/vmlinux.lds.S
index 2438b89a4620..9b8b70057bcf 100644
--- a/arch/x86/kernel/vmlinux.lds.S
+++ b/arch/x86/kernel/vmlinux.lds.S
@@ -155,6 +155,7 @@ SECTIONS
 #endif
 		ALIGN_ENTRY_TEXT_END
 
+		CPU_PRESERVED_TEXT
 		TEXT_TEXT
 		SCHED_TEXT
 		LOCK_TEXT
@@ -194,6 +195,7 @@ SECTIONS
 #endif
 
 		PAGE_ALIGNED_DATA(PAGE_SIZE)
+		CPU_PRESERVED_DATA
 
 		CACHE_HOT_DATA(L1_CACHE_BYTES)
 
-- 
2.55.0.1082.g2b9226bbc0-goog