memdisk: "safe hook" and mBFT

Two additions to MEMDISK to support OS drivers.

The "safe hook" structure ("Safe Master Boot Record INT 13h Hook Routines")
is a means for an OS driver to follow a chain of INT 13h hooks, examining
the hooks' vendors and assuming responsibility for hook functionality along
the way.  For MEMDISK, we guarantee an additional field which holds the
physical address for the mBFT.

The mBFT is an ACPI table which an OS driver can scan for.  The mBFT
contains the official MEMDISK Info structure (MDI) which itself includes
parameters the OS will want to know about.  The mBFT points back at the
"safe hook" structure's physical address so that an OS supporting both
"safe hook" chain-walking as well as mBFT-scanning can know that both
refer to the same MEMDISK instance.

Signed-off-by: Shao Miller <shao.miller@yrdsb.edu.on.ca>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 40 insertions, 1 deletions

diff --git a/memdisk/setup.c b/memdisk/setup.c
index db986faa..ea17afda 100644
--- a/memdisk/setup.c
+++ b/memdisk/setup.c
@@ -2,7 +2,7 @@
  *
  *   Copyright 2001-2009 H. Peter Anvin - All Rights Reserved
  *   Copyright 2009 Intel Corporation; author: H. Peter Anvin
- *   Portions copyright 2009 Shao Miller [El Torito code]
+ *   Portions copyright 2009 Shao Miller [El Torito code, mBFT, safe hook]
  *
  *   This program is free software; you can redistribute it and/or modify
  *   it under the terms of the GNU General Public License as published by
@@ -13,6 +13,7 @@
  * ----------------------------------------------------------------------- */
 
 #include <stdint.h>
+#include "acpi.h"
 #include "bda.h"
 #include "dskprobe.h"
 #include "e820.h"
@@ -47,6 +48,18 @@ struct memdisk_header {
     uint16_t iret_offs;
 };
 
+struct safe_hook {
+    uint8_t jump[3];		/* Max. three bytes for jump */
+    uint8_t signature[8];	/* "$INT13SF" */
+    uint8_t vendor[8];		/* "MEMDISK " */
+    uint32_t old_hook;		/* SEG:OFF for previous INT 13h hook */
+    uint32_t flags;		/* "Safe hook" flags */
+    /* The next field is a MEMDISK extension to the "safe hook" structure */
+    uint32_t mBFT;		/* Offset from hook to the mBFT; refilled
+				 * by setup() with the physical address
+				 */
+} __attribute__((packed));
+
 /* The Disk Parameter Table may be required */
 typedef union {
     struct hd_dpt {
@@ -102,6 +115,11 @@ struct edd_dpt {
     uint8_t  chksum;		/* DPI checksum */
 } __attribute__((packed));
 
+struct mBFT {
+    struct acpi_description_header acpi;
+    uint32_t safe_hook;		/* "Safe hook" physical address */
+} __attribute__((packed));
+
 struct patch_area {
     uint32_t diskbuf;
     uint32_t disksize;
@@ -816,6 +834,7 @@ void setup(const struct real_mode_args *rm_args_ptr)
     unsigned int bin_size;
     char *memdisk_hook;
     struct memdisk_header *hptr;
+    struct safe_hook *safe_hook;
     struct patch_area *pptr;
     uint16_t driverseg;
     uint32_t driverptr, driveraddr;
@@ -899,6 +918,7 @@ void setup(const struct real_mode_args *rm_args_ptr)
 
     /* Figure out where it needs to go */
     hptr = (struct memdisk_header *)memdisk_hook;
+    safe_hook = (struct safe_hook *)(memdisk_hook + hptr->int13_offs);
     pptr = (struct patch_area *)(memdisk_hook + hptr->patch_offs);
 
     dosmem_k = rdz_16(BIOS_BASEMEM);
@@ -1146,6 +1166,9 @@ void setup(const struct real_mode_args *rm_args_ptr)
 	}
     }
 
+    /* Note the previous INT 13h hook in the "safe hook" structure */
+    safe_hook->old_hook = pptr->oldint13;
+
     /* Add ourselves to the drive count */
     pptr->drivecnt++;
 
@@ -1170,6 +1193,7 @@ void setup(const struct real_mode_args *rm_args_ptr)
 
 	/* Adjust these pointers to point to the installed image */
 	/* Careful about the order here... the image isn't copied yet! */
+	safe_hook = (struct safe_hook *)(dpp + hptr->int13_offs);
 	pptr = (struct patch_area *)(dpp + hptr->patch_offs);
 	hptr = (struct memdisk_header *)dpp;
 
@@ -1179,6 +1203,21 @@ void setup(const struct real_mode_args *rm_args_ptr)
 	dpp = mempcpy(dpp, shdr->cmdline, cmdline_len);
     }
 
+    /* Re-fill the "safe hook" mBFT field with the physical address */
+    safe_hook->mBFT += (uint32_t)hptr;
+
+    /* Complete the mBFT */
+    {
+	struct mBFT *mBFT = (struct mBFT *)safe_hook->mBFT;
+
+	mBFT->acpi.signature[0] = 'm';	/* "mBFT" */
+	mBFT->acpi.signature[1] = 'B';
+	mBFT->acpi.signature[2] = 'F';
+	mBFT->acpi.signature[3] = 'T';
+	mBFT->safe_hook = (uint32_t)safe_hook;
+	mBFT->acpi.checksum = -checksum_buf(mBFT, mBFT->acpi.length);
+    }
+
     /* Update various BIOS magic data areas (gotta love this shit) */
 
     if (geometry->driveno & 0x80) {