x86/kexec: Consolidate relocate_kernel() function parameters

During kexec, the kernel jumps to the new kernel in relocate_kernel(),
which is implemented in assembly and both 32-bit and 64-bit have their
own version.

Currently, for both 32-bit and 64-bit, the last two parameters of the
relocate_kernel() are both 'unsigned int' but actually they only convey
a boolean, i.e., one bit information.  The 'unsigned int' has enough
space to carry two bits information therefore there's no need to pass
the two booleans in two separate 'unsigned int'.

Consolidate the last two function parameters of relocate_kernel() into a
single 'unsigned int' and pass flags instead.

Only consolidate the 64-bit version albeit the similar optimization can
be done for the 32-bit version too.  Don't bother changing the 32-bit
version while it is working (since assembly code change is required).

Signed-off-by: Kai Huang <kai.huang@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Link: https://lore.kernel.org/all/20250901160930.1785244-2-pbonzini%40redhat.com

Author: kaihuang

arch/x86/include/asm/kexec.h

@@ -13,6 +13,15 @@
 # define KEXEC_DEBUG_EXC_HANDLER_SIZE	6 /* PUSHI, PUSHI, 2-byte JMP */
 #endif
 
+#ifdef CONFIG_X86_64
+
+#include <linux/bits.h>
+
+#define RELOC_KERNEL_PRESERVE_CONTEXT		BIT(0)
+#define RELOC_KERNEL_HOST_MEM_ENC_ACTIVE	BIT(1)
+
+#endif
+
 # define KEXEC_CONTROL_PAGE_SIZE	4096
 # define KEXEC_CONTROL_CODE_MAX_SIZE	2048
 
@@ -121,8 +130,7 @@ typedef unsigned long
 relocate_kernel_fn(unsigned long indirection_page,
 		   unsigned long pa_control_page,
 		   unsigned long start_address,
-		   unsigned int preserve_context,
-		   unsigned int host_mem_enc_active);
+		   unsigned int flags);
 #endif
 extern relocate_kernel_fn relocate_kernel;
 #define ARCH_HAS_KIMAGE_ARCH

arch/x86/kernel/machine_kexec_64.c

@@ -384,16 +384,10 @@ void __nocfi machine_kexec(struct kimage *image)
 {
 	unsigned long reloc_start = (unsigned long)__relocate_kernel_start;
 	relocate_kernel_fn *relocate_kernel_ptr;
-	unsigned int host_mem_enc_active;
+	unsigned int relocate_kernel_flags;
 	int save_ftrace_enabled;
 	void *control_page;
 
-	/*
-	 * This must be done before load_segments() since if call depth tracking
-	 * is used then GS must be valid to make any function calls.
-	 */
-	host_mem_enc_active = cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT);
-
 #ifdef CONFIG_KEXEC_JUMP
 	if (image->preserve_context)
 		save_processor_state();
@@ -427,6 +421,17 @@ void __nocfi machine_kexec(struct kimage *image)
 	 */
 	relocate_kernel_ptr = control_page + (unsigned long)relocate_kernel - reloc_start;
 
+	relocate_kernel_flags = 0;
+	if (image->preserve_context)
+		relocate_kernel_flags |= RELOC_KERNEL_PRESERVE_CONTEXT;
+
+	/*
+	 * This must be done before load_segments() since if call depth tracking
+	 * is used then GS must be valid to make any function calls.
+	 */
+	if (cc_platform_has(CC_ATTR_HOST_MEM_ENCRYPT))
+		relocate_kernel_flags |= RELOC_KERNEL_HOST_MEM_ENC_ACTIVE;
+
 	/*
 	 * The segment registers are funny things, they have both a
 	 * visible and an invisible part.  Whenever the visible part is
@@ -443,8 +448,7 @@ void __nocfi machine_kexec(struct kimage *image)
 	image->start = relocate_kernel_ptr((unsigned long)image->head,
 					   virt_to_phys(control_page),
 					   image->start,
-					   image->preserve_context,
-					   host_mem_enc_active);
+					   relocate_kernel_flags);
 
 #ifdef CONFIG_KEXEC_JUMP
 	if (image->preserve_context)

arch/x86/kernel/relocate_kernel_64.S

@@ -66,8 +66,7 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	 * %rdi indirection_page
 	 * %rsi pa_control_page
 	 * %rdx start address
-	 * %rcx preserve_context
-	 * %r8  host_mem_enc_active
+	 * %rcx flags: RELOC_KERNEL_*
 	 */
 
 	/* Save the CPU context, used for jumping back */
@@ -111,7 +110,7 @@ SYM_CODE_START_NOALIGN(relocate_kernel)
 	/* save indirection list for jumping back */
 	movq	%rdi, pa_backup_pages_map(%rip)
 
-	/* Save the preserve_context to %r11 as swap_pages clobbers %rcx. */
+	/* Save the flags to %r11 as swap_pages clobbers %rcx. */
 	movq	%rcx, %r11
 
 	/* setup a new stack at the end of the physical control page */
@@ -129,9 +128,8 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	/*
 	 * %rdi	indirection page
 	 * %rdx start address
-	 * %r8 host_mem_enc_active
 	 * %r9 page table page
-	 * %r11 preserve_context
+	 * %r11 flags: RELOC_KERNEL_*
 	 * %r13 original CR4 when relocate_kernel() was invoked
 	 */
 
@@ -204,7 +202,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	 * entries that will conflict with the now unencrypted memory
 	 * used by kexec. Flush the caches before copying the kernel.
 	 */
-	testq	%r8, %r8
+	testb	$RELOC_KERNEL_HOST_MEM_ENC_ACTIVE, %r11b
 	jz .Lsme_off
 	wbinvd
 .Lsme_off:
@@ -220,7 +218,7 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	movq	%cr3, %rax
 	movq	%rax, %cr3
 
-	testq	%r11, %r11	/* preserve_context */
+	testb	$RELOC_KERNEL_PRESERVE_CONTEXT, %r11b
 	jnz .Lrelocate
 
 	/*
@@ -273,7 +271,13 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	ANNOTATE_NOENDBR
 	andq	$PAGE_MASK, %r8
 	lea	PAGE_SIZE(%r8), %rsp
-	movl	$1, %r11d	/* Ensure preserve_context flag is set */
+	/*
+	 * Ensure RELOC_KERNEL_PRESERVE_CONTEXT flag is set so that
+	 * swap_pages() can swap pages correctly.  Note all other
+	 * RELOC_KERNEL_* flags passed to relocate_kernel() are not
+	 * restored.
+	 */
+	movl	$RELOC_KERNEL_PRESERVE_CONTEXT, %r11d
 	call	swap_pages
 	movq	kexec_va_control_page(%rip), %rax
 0:	addq	$virtual_mapped - 0b, %rax
@@ -321,7 +325,7 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	UNWIND_HINT_END_OF_STACK
 	/*
 	 * %rdi indirection page
-	 * %r11 preserve_context
+	 * %r11 flags: RELOC_KERNEL_*
 	 */
 	movq	%rdi, %rcx	/* Put the indirection_page in %rcx */
 	xorl	%edi, %edi
@@ -357,7 +361,8 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	movq	%rdi, %rdx    /* Save destination page to %rdx */
 	movq	%rsi, %rax    /* Save source page to %rax */
 
-	testq	%r11, %r11    /* Only actually swap for ::preserve_context */
+	/* Only actually swap for ::preserve_context */
+	testb	$RELOC_KERNEL_PRESERVE_CONTEXT, %r11b
 	jz	.Lnoswap
 
 	/* copy source page to swap page */