doc: add note about KVM restriction on CS.base

Add note about KVM restrictions on CS.base before entering real mode.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

diff --git a/doc/comboot.txt b/doc/comboot.txt
index 9c81c48..68f1e7e 100644 (file)
--- a/doc/comboot.txt
+++ b/doc/comboot.txt
@@ -947,7 +947,9 @@ AX=0024h [3.80] Cleanup, shuffle and boot, raw version
 
        For mode 0 (16-bit real mode), EAX will contain CR0 with bit 0
        masked out, suitable for loading into CR0 to immediately enter
-       real mode.
+       real mode.  Note: if real-mode entry is planned,
+       (CS.base & 0xfff0000f) == 0 for compatibility with KVM, and
+       possibly other virtualization solutions.
 
        In both mode 0 and mode 1, the data segments will be loaded
        with base-zero read/write segments.  For mode 0, B=0 and the