x86/EFI: make trampoline allocation more flexible

Certain UEFI implementations reserve all memory below 1Mb at boot time,
making it impossible to properly allocate the chunk necessary for the
trampoline. Fall back to simply grabbing a chunk from EfiBootServices*
regions immediately prior to calling ExitBootServices().

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/efi/boot.c
+++ b/xen/arch/x86/efi/boot.c
@@ -746,6 +746,22 @@ static void __init relocate_image(unsign
 extern const s32 __trampoline_rel_start[], __trampoline_rel_stop[];
 extern const s32 __trampoline_seg_start[], __trampoline_seg_stop[];
 
+static void __init relocate_trampoline(unsigned long phys)
+{
+    const s32 *trampoline_ptr;
+
+    trampoline_phys = phys;
+    /* Apply relocations to trampoline. */
+    for ( trampoline_ptr = __trampoline_rel_start;
+          trampoline_ptr < __trampoline_rel_stop;
+          ++trampoline_ptr )
+        *(u32 *)(*trampoline_ptr + (long)trampoline_ptr) += phys;
+    for ( trampoline_ptr = __trampoline_seg_start;
+          trampoline_ptr < __trampoline_seg_stop;
+          ++trampoline_ptr )
+        *(u16 *)(*trampoline_ptr + (long)trampoline_ptr) = phys >> 4;
+}
+
 void EFIAPI __init __attribute__((__noreturn__))
 efi_start(EFI_HANDLE ImageHandle, EFI_SYSTEM_TABLE *SystemTable)
 {
@@ -765,7 +781,6 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
     EFI_GRAPHICS_OUTPUT_MODE_INFORMATION *mode_info;
     EFI_FILE_HANDLE dir_handle;
     union string section = { NULL }, name;
-    const s32 *trampoline_ptr;
     struct e820entry *e;
     u64 efer;
     bool_t base_video = 0;
@@ -1268,23 +1283,13 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
     cfg.size = trampoline_end - trampoline_start;
     status = efi_bs->AllocatePages(AllocateMaxAddress, EfiLoaderData,
                                    PFN_UP(cfg.size), &cfg.addr);
-    if ( EFI_ERROR(status) )
+    if ( status == EFI_SUCCESS )
+        relocate_trampoline(cfg.addr);
+    else
     {
         cfg.addr = 0;
-        blexit(L"No memory for trampoline\r\n");
+        PrintStr(L"Trampoline space cannot be allocated; will try fallback.\r\n");
     }
-    trampoline_phys = cfg.addr;
-    /* Apply relocations to trampoline. */
-    for ( trampoline_ptr = __trampoline_rel_start;
-          trampoline_ptr < __trampoline_rel_stop;
-          ++trampoline_ptr )
-        *(u32 *)(*trampoline_ptr + (long)trampoline_ptr) +=
-            trampoline_phys;
-    for ( trampoline_ptr = __trampoline_seg_start;
-          trampoline_ptr < __trampoline_seg_stop;
-          ++trampoline_ptr )
-        *(u16 *)(*trampoline_ptr + (long)trampoline_ptr) =
-            trampoline_phys >> 4;
 
     /* Initialise L2 identity-map and boot-map page table entries (16MB). */
     for ( i = 0; i < 8; ++i )
@@ -1400,10 +1405,14 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
             type = E820_RESERVED;
             break;
         case EfiConventionalMemory:
-        case EfiLoaderCode:
-        case EfiLoaderData:
         case EfiBootServicesCode:
         case EfiBootServicesData:
+            if ( !trampoline_phys && desc->PhysicalStart + len <= 0x100000 &&
+                 len >= cfg.size && desc->PhysicalStart + len > cfg.addr )
+                cfg.addr = (desc->PhysicalStart + len - cfg.size) & PAGE_MASK;
+            /* fall through */
+        case EfiLoaderCode:
+        case EfiLoaderData:
             if ( desc->Attribute & EFI_MEMORY_WB )
                 type = E820_RAM;
             else
@@ -1431,6 +1440,12 @@ efi_start(EFI_HANDLE ImageHandle, EFI_SY
             ++e820nr;
         }
     }
+    if ( !trampoline_phys )
+    {
+        if ( !cfg.addr )
+            blexit(L"No memory for trampoline");
+        relocate_trampoline(cfg.addr);
+    }
 
     status = efi_bs->ExitBootServices(ImageHandle, map_key);
     if ( EFI_ERROR(status) )