[ipxe-devel] [PATCH] wimboot syslinux kludge 1/4: add efihandover protocol support

[Friedemann Gerold]

Wimboot cannot be started from syslinux on a EFI system, as starting
EFI applications is not supported by syslinux.

However, syslinux can start a linux kernel and pass on the EFI system
table and image handle using a special linux specific entry point.

This patch allows syslinux to start wimboot under EFI on AMD64,
by having wimboot masquerade as a linux bzImage supporting the
efi handover protocol.

Signed-off-by: Friedemann Gerold <f.gerold at b-c-s.de>
---
--- wimbootorig/src/prefix.S	2018-11-02 12:29:14.343297008 +0100
+++ wimboot/src/prefix.S	2018-11-02 12:28:48.071166732 +0100
@@ -33,8 +33,12 @@
  #define i386(symbol) symbol
  #endif

-/** Standard number of setup sectors */
-#define SETUP_SECTS 4
+/*
+ * Standard number of setup sectors was 5,
+ * but we need to round it to 4k to page
+ * align _payload.
+ */
+#define SETUP_SECTS ((0x1000/SECTOR_SIZE)-1)

  /** Sector size */
  #define SECTOR_SIZE 512
@@ -219,7 +223,7 @@

  	.org	0x206
  version:
-	.word	0x203	/* Version 2.03 */
+	.word	0x20b	/* Version 2.11 */

  	.org	0x20e
  kernel_version:
@@ -229,6 +233,10 @@
  loadflags:
  	.byte	LOADED_HIGH

+	.org	0x214
+code32_start:
+	.long	0
+
  	.org	0x218
  ramdisk_image:
  	.long	0	/* Filled in by boot loader */
@@ -245,6 +253,37 @@
  ramdisk_max:
  	.long	0x7fffffff

+	.org	0x236
+xloadflags:
+	.word	(1<<3)
+
+	.org	0x238
+cmdline_size:
+	.long	256
+
+	.org	0x23c
+hardware_subarch:
+	.long	0
+
+	.org	0x258
+prefaddr:
+	.long	_payload
+	.long	0
+
+	.org	0x260
+init_size:
+	.long	0x10000
+
+	.org	0x264
+handover_offset:
+	/*
+	 * really would like to write efihandover-_payload here,
+	 * but we cant as efihandover its part of the payload
+	 * section.  0x4000 is the page rounded size of this
+	 * prefix setion (4k) + text16/bss16 section (12k).
+	 */
+	.long ( efihandover-BASE_ADDRESS-0x4000 )
+
  version_string:
  	.asciz	VERSION

--- wimbootorig/src/startup.S	2018-11-02 12:29:14.343297008 +0100
+++ wimboot/src/startup.S	2018-11-02 12:28:48.071166732 +0100
@@ -57,6 +57,15 @@
  	jmp	reboot
  	.size	startup, . - startup

+	.section ".text", "ax", @progbits
+	.globl	efihandover
+	.globl	efihandover64
+efihandover:
+        .org    .+512, 0x90
+efihandover64:
+	jmp	efi_linuxentry
+	.size	startup, . - startup
+
  	/* Reboot system */
  	.section ".text", "ax", @progbits
  	.globl	reboot
--- wimbootorig/src/efimain.c	2018-11-02 12:29:14.343297008 +0100
+++ wimboot/src/efimain.c	2018-11-02 13:41:05.044672622 +0100
@@ -99,3 +99,19 @@

  	return 0;
  }
+
+void
+efi_linuxentry(EFI_HANDLE image_handle, EFI_SYSTEM_TABLE *systab, uint32_t *bp)
+{
+	efi_image_handle = image_handle;
+	efi_systab = systab;
+
+#if __x86_64__
+	extern char _bss[];
+	extern char _ebss[];
+	memset(_bss, 0, _ebss-_bss);
+#endif
+
+	efi_main(image_handle, systab);
+}
+