[xen staging] x86/boot: Explain how moving mod[0] works

[patchbot@xxxxxxx]

commit 49c810e86799211bf15e67ddff8dca4e29166ca2
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Apr 23 16:45:36 2024 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu May 9 18:19:49 2024 +0100

    x86/boot: Explain how moving mod[0] works
    
    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>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
    Reviewed-by: Jason Andryuk <jason.andryuk@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 caf235c628..f84e1cd79c 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 incorporate this as extra space at the start.
+             */
             unsigned long headroom = j ? 0 : modules_headroom;
             unsigned long size = PAGE_ALIGN(headroom + mod[j].mod_end);
 
--
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