md: use prestaged mfs_root

On PowerNV systems, the rootfs is passed through kexec, which loads the rootfs
into memory and set two fdt entries to describe where the file is located in
the memory;

I need to pass this memory region to the md device as a mfs_root, but, current
md driver does not support two things:

 * Just getting a pointer from an external (bootloader) memory. If I need to
workaround it, I would need to declare a static array and memcopy from this
external memory to this static variable.

 * The size of the image. The usage of mfs_root_end, which is not a pointer,
seems to be not possible for this prestaged scenario.

This patch simply adds a new way to load mfs_root from memory.

Differential Revision: https://reviews.freebsd.org/D15625
Approved by: kib, jhibbits (mentor)

1 files changed, 8 insertions, 0 deletions

diff --git a/sys/dev/md/md.c b/sys/dev/md/md.c
index be661b2703c7..2209c6680be5 100644
--- a/sys/dev/md/md.c
+++ b/sys/dev/md/md.c
@@ -180,6 +180,10 @@ SYSCTL_INT(_vm, OID_AUTO, md_malloc_wait, CTLFLAG_RW, &md_malloc_wait, 0,
  */
 u_char mfs_root[MD_ROOT_SIZE*1024] __attribute__ ((section ("oldmfs")));
 const int mfs_root_size = sizeof(mfs_root);
+#elif defined(MD_ROOT_MEM)
+/* MD region already mapped in the memory */
+u_char *mfs_root;
+int mfs_root_size;
 #else
 extern volatile u_char __weak_symbol mfs_root;
 extern volatile u_char __weak_symbol mfs_root_end;
@@ -2074,8 +2078,12 @@ g_md_init(struct g_class *mp __unused)
 #ifdef MD_ROOT
 	if (mfs_root_size != 0) {
 		sx_xlock(&md_sx);
+#ifdef MD_ROOT_MEM
+		md_preloaded(mfs_root, mfs_root_size, NULL);
+#else
 		md_preloaded(__DEVOLATILE(u_char *, &mfs_root), mfs_root_size,
 		    NULL);
+#endif
 		sx_xunlock(&md_sx);
 	}
 #endif