[PATCH 1/3] x86/boot: Explain how moving mod[0] works

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

modules_headroom is a misleading name as it applies strictly to mod[0] only,
and the movement loop is deeply unintuitive and completely undocumented.

Provide help to whomever needs to look at this code next.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
CC: Daniel Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
CC: Christopher Clark <christopher.w.clark@xxxxxxxxx>
---
 xen/arch/x86/setup.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/xen/arch/x86/setup.c b/xen/arch/x86/setup.c
index caf235c6286d..59907fae095f 100644
--- a/xen/arch/x86/setup.c
+++ b/xen/arch/x86/setup.c
@@ -1432,6 +1432,11 @@ void asmlinkage __init noreturn __start_xen(unsigned 
long mbi_p)
         /* Is the region suitable for relocating the multiboot modules? */
         for ( j = mbi->mods_count - 1; j >= 0; j-- )
         {
+            /*
+             * 'headroom' is a guess for the decompressed size and
+             * decompressor overheads of mod[0] (the dom0 kernel).  When we
+             * move mod[0], we incorperate this as extra space at the start.
+             */
             unsigned long headroom = j ? 0 : modules_headroom;
             unsigned long size = PAGE_ALIGN(headroom + mod[j].mod_end);
 
-- 
2.30.2