Add docs/pvh.md

Brief description of the PVH boot mode.  We defer to Xen for technical
details of how CPU registers are set up upon kernel entry.

Signed-off-by: Colin Percival <[email protected]>

Author: cperciva

docs/pvh.md

@@ -0,0 +1,15 @@
+# PVH boot mode
+
+Firecracker supports booting x86 kernels in "PVH direct boot" mode
+[as specified by the Xen project](https://github.com/xen-project/xen/blob/master/docs/misc/pvh.pandoc).
+If a kernel is provided which contains the XEN_ELFNOTE_PHYS32_ENTRY ELF Note
+then this boot mode will be used.  This boot mode was designed for virtualized
+environments which load the kernel directly, and is simpler than the "Linux
+boot" mode which is designed to be launched from a legacy boot loader.
+
+PVH boot mode can be enabled for Linux by setting CONFIG_XEN_PVH=y in the
+kernel configuration.  (This is not the default setting.)
+
+PVH boot mode is enabled by default in FreeBSD, which has support for
+Firecracker starting with FreeBSD 14.0.  Instructions on building a FreeBSD
+kernel and root filesystem are available [here](rootfs-and-kernel-setup.md).