[Qemu-devel] [PATCH] Allow -kernel without -hda

["Ed Swierk" <eswierk@arastra.com>]

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The qemu -kernel option currently requires specifying a hard disk
image with -hda. Ostensibly at least one hard disk is needed for
qemu's boot loader to populate the partition table in its array of
boot sectors.

Passing -hda /dev/zero tricks qemu into booting, which demonstrates
that the requirement is unnecessary. Booting with no disk image is
needed to support diskless configurations where a remote NFS directory
is used as the root filesystem. In this scenario, the user invokes
qemu with -kernel and -initrd options, with a specially configured
initrd that NFS-mounts a filesystem on / before passing control to the
real init.

The attached patch permits using the -kernel option with no disk
images, and skips copying the partition table in this case.

--Ed

[-- Attachment #2: qemu-kernel-without-hda.patch --]
[-- Type: text/x-patch, Size: 1399 bytes --]

diff -BurN qemu-snapshot-2006-03-27_23.orig/hw/pc.c qemu-snapshot-2006-03-27_23/hw/pc.c
--- qemu-snapshot-2006-03-27_23.orig/hw/pc.c	2006-04-05 13:05:17.000000000 +0000
+++ qemu-snapshot-2006-03-27_23/hw/pc.c	2006-04-05 13:12:40.000000000 +0000
@@ -707,10 +707,6 @@
         uint8_t bootsect[512];
         uint8_t old_bootsect[512];
 
-        if (bs_table[0] == NULL) {
-            fprintf(stderr, "A disk image must be given for 'hda' when booting a Linux kernel\n");
-            exit(1);
-        }
         snprintf(buf, sizeof(buf), "%s/%s", bios_dir, LINUX_BOOT_FILENAME);
         ret = load_image(buf, bootsect);
         if (ret != sizeof(bootsect)) {
@@ -719,12 +715,14 @@
             exit(1);
         }
 
-        if (bdrv_read(bs_table[0], 0, old_bootsect, 1) >= 0) {
-            /* copy the MSDOS partition table */
-            memcpy(bootsect + 0x1be, old_bootsect + 0x1be, 0x40);
-        }
+        if (bs_table[0]) {
+            if (bdrv_read(bs_table[0], 0, old_bootsect, 1) >= 0) {
+                /* copy the MSDOS partition table */
+                memcpy(bootsect + 0x1be, old_bootsect + 0x1be, 0x40);
+            }
 
-        bdrv_set_boot_sector(bs_table[0], bootsect, sizeof(bootsect));
+            bdrv_set_boot_sector(bs_table[0], bootsect, sizeof(bootsect));
+        }
 
         /* now we can load the kernel */
         ret = load_kernel(kernel_filename,