x86/decompressor: Call trampoline directly from C code

Instead of returning to the asm calling code to invoke the trampoline,
call it straight from the C code that sets it up. That way, the struct
return type is no longer needed for returning two values, and the call
can be made conditional more cleanly in a subsequent patch.

This means that all callee save 64-bit registers need to be preserved
and restored, as their contents may not survive the legacy mode switch.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/r/20230807162720.545787-13-ardb@kernel.org

Author: ardbiesheuvel

arch/x86/boot/compressed/head_64.S

@@ -430,25 +430,14 @@ SYM_CODE_START(startup_64)
 #endif
 
 	/*
-	 * paging_prepare() sets up the trampoline and checks if we need to
-	 * enable 5-level paging.
-	 *
-	 * paging_prepare() returns a two-quadword structure which lands
-	 * into RDX:RAX:
-	 *   - Address of the trampoline is returned in RAX.
-	 *   - Non zero RDX means trampoline needs to enable 5-level
-	 *     paging.
+	 * configure_5level_paging() updates the number of paging levels using
+	 * a trampoline in 32-bit addressable memory if the current number does
+	 * not match the desired number.
 	 *
 	 * Pass the boot_params pointer as the first argument.
 	 */
 	movq	%r15, %rdi
-	call	paging_prepare
-
-	/* Pass the trampoline address and boolean flag as args #1 and #2 */
-	movq	%rax, %rdi
-	movq	%rdx, %rsi
-	leaq	TRAMPOLINE_32BIT_CODE_OFFSET(%rax), %rax
-	call	*%rax
+	call	configure_5level_paging
 
 	/*
 	 * cleanup_trampoline() would restore trampoline memory.
@@ -543,11 +532,14 @@ SYM_FUNC_END(.Lrelocated)
 	.section ".rodata", "a", @progbits
 SYM_CODE_START(trampoline_32bit_src)
 	/*
-	 * Preserve live 64-bit registers on the stack: this is necessary
-	 * because the architecture does not guarantee that GPRs will retain
-	 * their full 64-bit values across a 32-bit mode switch.
+	 * Preserve callee save 64-bit registers on the stack: this is
+	 * necessary because the architecture does not guarantee that GPRs will
+	 * retain their full 64-bit values across a 32-bit mode switch.
 	 */
 	pushq	%r15
+	pushq	%r14
+	pushq	%r13
+	pushq	%r12
 	pushq	%rbp
 	pushq	%rbx
 
@@ -574,6 +566,9 @@ SYM_CODE_START(trampoline_32bit_src)
 	/* Restore the preserved 64-bit registers */
 	popq	%rbx
 	popq	%rbp
+	popq	%r12
+	popq	%r13
+	popq	%r14
 	popq	%r15
 	retq
 

arch/x86/boot/compressed/pgtable_64.c

@@ -16,11 +16,6 @@ unsigned int __section(".data") pgdir_shift = 39;
 unsigned int __section(".data") ptrs_per_p4d = 1;
 #endif
 
-struct paging_config {
-	unsigned long trampoline_start;
-	unsigned long l5_required;
-};
-
 /* Buffer to preserve trampoline memory */
 static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
 
@@ -29,7 +24,7 @@ static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
  * purposes.
  *
  * Avoid putting the pointer into .bss as it will be cleared between
- * paging_prepare() and extract_kernel().
+ * configure_5level_paging() and extract_kernel().
  */
 unsigned long *trampoline_32bit __section(".data");
 
@@ -106,13 +101,13 @@ static unsigned long find_trampoline_placement(void)
 	return bios_start - TRAMPOLINE_32BIT_SIZE;
 }
 
-struct paging_config paging_prepare(void *rmode)
+asmlinkage void configure_5level_paging(struct boot_params *bp)
 {
-	struct paging_config paging_config = {};
-	void *tramp_code;
+	void (*toggle_la57)(void *trampoline, bool enable_5lvl);
+	bool l5_required = false;
 
 	/* Initialize boot_params. Required for cmdline_find_option_bool(). */
-	boot_params = rmode;
+	boot_params = bp;
 
 	/*
 	 * Check if LA57 is desired and supported.
@@ -130,17 +125,15 @@ struct paging_config paging_prepare(void *rmode)
 			!cmdline_find_option_bool("no5lvl") &&
 			native_cpuid_eax(0) >= 7 &&
 			(native_cpuid_ecx(7) & (1 << (X86_FEATURE_LA57 & 31)))) {
-		paging_config.l5_required = 1;
+		l5_required = true;
 
 		/* Initialize variables for 5-level paging */
 		__pgtable_l5_enabled = 1;
 		pgdir_shift = 48;
 		ptrs_per_p4d = 512;
 	}
 
-	paging_config.trampoline_start = find_trampoline_placement();
-
-	trampoline_32bit = (unsigned long *)paging_config.trampoline_start;
+	trampoline_32bit = (unsigned long *)find_trampoline_placement();
 
 	/* Preserve trampoline memory */
 	memcpy(trampoline_save, trampoline_32bit, TRAMPOLINE_32BIT_SIZE);
@@ -149,7 +142,7 @@ struct paging_config paging_prepare(void *rmode)
 	memset(trampoline_32bit, 0, TRAMPOLINE_32BIT_SIZE);
 
 	/* Copy trampoline code in place */
-	tramp_code = memcpy(trampoline_32bit +
+	toggle_la57 = memcpy(trampoline_32bit +
 			TRAMPOLINE_32BIT_CODE_OFFSET / sizeof(unsigned long),
 			&trampoline_32bit_src, TRAMPOLINE_32BIT_CODE_SIZE);
 
@@ -159,7 +152,8 @@ struct paging_config paging_prepare(void *rmode)
 	 * immediate absolute address, which needs to be adjusted based on the
 	 * placement of the trampoline.
 	 */
-	*(u32 *)(tramp_code + trampoline_ljmp_imm_offset) += (unsigned long)tramp_code;
+	*(u32 *)((u8 *)toggle_la57 + trampoline_ljmp_imm_offset) +=
+						(unsigned long)toggle_la57;
 
 	/*
 	 * The code below prepares page table in trampoline memory.
@@ -175,10 +169,10 @@ struct paging_config paging_prepare(void *rmode)
 	 * We are not going to use the page table in trampoline memory if we
 	 * are already in the desired paging mode.
 	 */
-	if (paging_config.l5_required == !!(native_read_cr4() & X86_CR4_LA57))
+	if (l5_required == !!(native_read_cr4() & X86_CR4_LA57))
 		goto out;
 
-	if (paging_config.l5_required) {
+	if (l5_required) {
 		/*
 		 * For 4- to 5-level paging transition, set up current CR3 as
 		 * the first and the only entry in a new top-level page table.
@@ -201,7 +195,7 @@ struct paging_config paging_prepare(void *rmode)
 	}
 
 out:
-	return paging_config;
+	toggle_la57(trampoline_32bit, l5_required);
 }
 
 void cleanup_trampoline(void *pgtable)