x86/boot: Explain how discard_initial_images() works

discard_initial_images() only works because init_domheap_pages() with ps==pe
is a no-op.

In dom0_construct(), explaining the significance of setting the initrd length
to 0, and put an explicit check in discard_initial_images().

No functional change.

Signed-off-by: Andrew Cooper <[email protected]>
Reviewed-by: Daniel P. Smith <[email protected]>

xen/arch/x86/pv/dom0_build.c

@@ -641,6 +641,15 @@ static int __init dom0_construct(struct domain *d,
                 if ( assign_pages(mfn_to_page(_mfn(mfn++)), 1, d, 0) )
                     BUG();
         }
+
+        /*
+         * We have either:
+         * - Mapped the initrd directly into dom0, or
+         * - Copied it and freed the module.
+         *
+         * Either way, tell discard_initial_images() to not free it a second
+         * time.
+         */
         initrd->mod_end = 0;
 
         iommu_memory_setup(d, "initrd", mfn_to_page(_mfn(initrd_mfn)),

xen/arch/x86/setup.c

@@ -340,17 +340,24 @@ unsigned long __init initial_images_nrpages(nodeid_t node)
     return nr;
 }
 
-void __init discard_initial_images(void)
+void __init discard_initial_images(void) /* a.k.a. Free boot modules */
 {
     struct boot_info *bi = &xen_boot_info;
     unsigned int i;
 
     for ( i = 0; i < bi->nr_modules; ++i )
     {
         uint64_t start = pfn_to_paddr(bi->mods[i].mod->mod_start);
+        uint64_t size  = bi->mods[i].mod->mod_end;
 
-        init_domheap_pages(start,
-                           start + PAGE_ALIGN(bi->mods[i].mod->mod_end));
+        /*
+         * Sometimes the initrd is mapped, rather than copied, into dom0.
+         * Size being 0 is how we're instructed to leave the module alone.
+         */
+        if ( size == 0 )
+            continue;
+
+        init_domheap_pages(start, start + PAGE_ALIGN(size));
     }
 
     bi->nr_modules = 0;