head: don't test for boot protocol, use hand-off that works for all

Linux, Multiboot2 and simple payload don't have overlapping uses for
values passed through registers or stack. All of those can be set at
the same time.

Signed-off-by: Krystian Hebel <[email protected]>

Author: krystian-hebel

head.S

@@ -195,13 +195,12 @@ GLOBAL(_entry)
 	 * skl_main() is magic.  It returns two pointers by register:
 	 *
 	 * %eax - protected mode kernel entry
-	 * %edx - ZP base
+	 * %edx - argument for kernel entry point, depends on type of kernel
 	 *
-	 * We stash the entry point in %ebx and the Zero Page pointer in %esi.
-	 * This both protects them from clobbering during teardown, and
-	 * matches the ABI for entering Linux.
+	 * We stash the entry point in %edi and the argument in %esi to protect
+	 * them from clobbering during teardown.
 	 */
-	mov	%eax, %ebx
+	mov	%eax, %edi
 	mov	%edx, %esi
 
 #ifdef __x86_64__
@@ -235,28 +234,28 @@ GLOBAL(_entry)
 	push	$0
 	popf
 
-	mov	boot_protocol(%ebp), %edx
-	cmp	$MULTIBOOT2, %edx
-	je	multiboot2_kernel
-
-	cmp	$SIMPLE_PAYLOAD, %edx
-	je	simple_payload
-
-	/* Jump to entry target - EBX: startup_32, ESI: ZP base, EDX: SKL base */
-	mov	%ebp, %edx
-	jmp	*%ebx
-
-multiboot2_kernel:
-	/* Jump to entry target - EBX: MBI pointer, EAX: Multiboot2 magic number */
-	xchg	%esi, %ebx
+	/*
+	 * Various kernels use different boot protocols, SKL supports some of
+	 * the common ones. Because of that, we are saving the same argument in
+	 * every possible place that any of the supported kernel types may look
+	 * for it. As of now, supported protocols include:
+	 *
+	 * - Linux x86 protected mode entry, not UEFI
+	 * - Multiboot2, also not UEFI
+	 * - simple payload started as 'entry(u32 arg)' function call. As we
+	 *   don't expect it to return, __cdecl, __stdcall and __pascal calling
+	 *   conventions work the same.
+	 */
+	/* Linux expects Zero Page address in %esi, it is already there */
+	/* Multiboot2 expects MBI address in %ebx and magic number in %eax */
+	mov	%esi, %ebx
 	mov	$MULTIBOOT2_BOOTLOADER_MAGIC, %eax
-	jmp *%esi
-
-simple_payload:
-	/* Jump to entry target - argument and dummy return address on stack */
+	/* Simple payload expects argument on stack followed by return address */
 	push	%esi
 	push	$0
-	jmp	*%ebx
+
+	/* All set, jump to the kernel */
+	jmp	*%edi
 ENDFUNC(_entry)
 
 	.section .rodata, "a", @progbits

include/defs.h

@@ -23,10 +23,6 @@
  */
 #pragma GCC visibility push(hidden)
 
-#define LINUX_BOOT      0
-#define SIMPLE_PAYLOAD  1
-#define MULTIBOOT2      2
-
 /* Update if code changes significantly. */
 #define MAX_STACK_SIZE  0x280
 

main.c

@@ -33,8 +33,6 @@
 #include <printk.h>
 #include <dev.h>
 
-u32 boot_protocol;
-
 const skl_info_t __used skl_info = {
     .uuid = {
         0x78, 0xf1, 0x26, 0x8e, 0x04, 0x92, 0x11, 0xe9,
@@ -438,17 +436,13 @@ static asm_return_t skl_multiboot2(struct tpm *tpm, struct skl_tag_boot_mb2 *skl
         reboot();
     }
 
-    boot_protocol = MULTIBOOT2;
-
     return (asm_return_t){ kernel_entry, _p(skl_tag->mbi) };
 }
 
 static asm_return_t skl_simple_payload(struct tpm *tpm, struct skl_tag_boot_simple_payload *skl_tag)
 {
     extend_pcr(tpm, _p(skl_tag->base), skl_tag->size, 17, "Measured payload into PCR17");
 
-    boot_protocol = SIMPLE_PAYLOAD;
-
     return (asm_return_t){ _p(skl_tag->entry), _p(skl_tag->arg) };
 }