[PATCH v2 1/2] x86/boot: Avoid relocations in trampoline code to physical addresses

[Frediano Ziglio <frediano.ziglio@xxxxxxxxx>]

The trampoline could have "manual" relocation entries (created
by assembly macros and some code to use them) and (in case of PE)
normal executable relocations.
Remove some normal executable ones. In this way we don't have to
worry about applying both correctly (they need proper order
which is hard to spot looking at the code).
Specifically in efi_arch_post_exit_boot trampoline is copied after
fixing relocations with efi_arch_relocate_image. These time dependencies
between different part of code are hard to spot making hard to change
code. In this case the copy is done in a state where code should be run
at higher locations so it would be better to reduce the code between
calling efi_arch_relocate_image and jumping to higher location.
Absolute symbols are defined by linker in order to avoid relocations.
These symbols use a "_PA" suffix to avoid possible clashes.
phys_addr macro is used to make more clear the address we want and making
symbol search easier.
sym_offs and sym_esi are undefined to avoid the usage of them which
would lead to relocations.

Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>
---
 xen/arch/x86/boot/head.S       | 6 ++++--
 xen/arch/x86/boot/trampoline.S | 9 ++++++++-
 xen/arch/x86/boot/wakeup.S     | 4 ++--
 xen/arch/x86/xen.lds.S         | 9 +++++++--
 4 files changed, 21 insertions(+), 7 deletions(-)
---
Changes since v1:
- add subject prefix to commit messages;
- more explanation of the improvement;
- use upper case for suffix to avoid clashes;
- undefine some macros to avoid usage of them;
- use some macros to make code more clear;
- use hidden symbols;
- split change in 2 commits.

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index d8ac0f0494..a8a14060b6 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -870,8 +870,10 @@ cmdline_parse_early:
 reloc:
         .incbin "reloc.bin"
 
+#include "x86_64.S"
+
+        .section .init.text, "ax", @progbits
+
 ENTRY(trampoline_start)
 #include "trampoline.S"
 ENTRY(trampoline_end)
-
-#include "x86_64.S"
diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index b8ab0ffdcb..3a6eb942a7 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -16,6 +16,11 @@
  * not guaranteed to persist.
  */
 
+/* We don't want relocation in the trampoline.
+ * Undefine some macros generating relocations. */
+#undef sym_offs
+#undef sym_esi
+
 /* NB. bootsym() is only usable in real mode, or via BOOT_PSEUDORM_DS. */
 #undef bootsym
 #define bootsym(s) ((s)-trampoline_start)
@@ -34,6 +39,8 @@
         .long 111b - (off) - .;            \
         .popsection
 
+#define phys_addr(sym) sym ## _PA
+
 /* Start of the permanent trampoline code. */
 
         .code16
@@ -73,7 +80,7 @@ trampoline_protmode_entry:
         mov     %ecx,%cr4
 
         /* Load pagetable base register. */
-        mov     $sym_offs(idle_pg_table),%eax
+        mov     $phys_addr(idle_pg_table), %eax
         add     bootsym_rel(trampoline_xen_phys_start,4,%eax)
         mov     %eax,%cr3
 
diff --git a/xen/arch/x86/boot/wakeup.S b/xen/arch/x86/boot/wakeup.S
index 08447e1934..ae4dd5eb40 100644
--- a/xen/arch/x86/boot/wakeup.S
+++ b/xen/arch/x86/boot/wakeup.S
@@ -99,7 +99,7 @@ wakeup_32:
         mov     $bootsym_rel(wakeup_stack, 4, %esp)
 
         # check saved magic again
-        mov     $sym_offs(saved_magic),%eax
+        mov     $phys_addr(saved_magic), %eax
         add     bootsym_rel(trampoline_xen_phys_start, 4, %eax)
         mov     (%eax), %eax
         cmp     $0x9abcdef0, %eax
@@ -112,7 +112,7 @@ wakeup_32:
         mov     %ecx, %cr4
 
         /* Load pagetable base register */
-        mov     $sym_offs(idle_pg_table),%eax
+        mov     $phys_addr(idle_pg_table), %eax
         add     bootsym_rel(trampoline_xen_phys_start,4,%eax)
         mov     %eax,%cr3
 
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index 9a1dfe1b34..5cfbd2524a 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -22,7 +22,7 @@ ENTRY(efi_start)
 
 #define FORMAT "elf64-x86-64"
 
-ENTRY(start_pa)
+ENTRY(start_PA)
 
 #endif /* EFI */
 
@@ -71,7 +71,12 @@ SECTIONS
   __2M_text_start = .;         /* Start of 2M superpages, mapped RX. */
 #endif
 
-  start_pa = ABSOLUTE(start - __XEN_VIRT_START);
+#define DEFINE_PA_ADDRESS(sym) \
+  HIDDEN(sym ## _PA = ABSOLUTE(sym - __XEN_VIRT_START))
+
+  DEFINE_PA_ADDRESS(start);
+  DEFINE_PA_ADDRESS(saved_magic);
+  DEFINE_PA_ADDRESS(idle_pg_table);
 
   . = __XEN_VIRT_START + XEN_IMG_OFFSET;
   _start = .;
-- 
2.46.0