[PATCH v4 1/3] x86: Put trampoline in separate .init.trampoline section

[Frediano Ziglio <frediano.ziglio@cloud.com>]

This change put the trampoline in a separate, not executable section.
The trampoline contains a mix of code and data (data which
is modified from C code during early start so must be writable).
This is in preparation for W^X patch in order to satisfy UEFI CA
memory mitigation requirements.
At the moment .init.text and .init.data in EFI mode are put together
so they will be in the same final section as before this patch.
Putting in a separate section (even in final executables) allows
to easily disassembly that section. As we need to have a writable
section and as we can't have code and data together to satisfy W^X
requirement we need to have a data section. However tools like objdump
by default do not disassemble data sections. Forcing disassembly of
data sections would result in a very large output and possibly crash
of tools. Putting in a separate section allows to selectively
disassemble that part of code using a command like

    objdump -m i386 -j .init.trampoline -d xen-syms

Signed-off-by: Frediano Ziglio <frediano.ziglio@cloud.com>
---
Changes since last version:
- use completely separate section even on final executables
  (suggested by Jan Beulich).

Changes since v1:
- remove useless align.

Changes since v2:
- remove change to alignment;
- improved commit message.

Changes since v3:
- split commit, add more requirements.
---
 xen/arch/x86/boot/head.S | 5 +++--
 xen/arch/x86/xen.lds.S   | 4 ++++
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 12bbb97f33..493286a9fb 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -882,8 +882,9 @@ cmdline_parse_early:
 reloc:
         .incbin "reloc.bin"
 
+#include "x86_64.S"
+
+        .section .init.trampoline, "aw", @progbits
 ENTRY(trampoline_start)
 #include "trampoline.S"
 ENTRY(trampoline_end)
-
-#include "x86_64.S"
diff --git a/xen/arch/x86/xen.lds.S b/xen/arch/x86/xen.lds.S
index d48de67cfd..22fb7d8458 100644
--- a/xen/arch/x86/xen.lds.S
+++ b/xen/arch/x86/xen.lds.S
@@ -269,6 +269,10 @@ SECTIONS
        __ctors_end = .;
   } PHDR(text)
 
+  DECL_SECTION(.init.trampoline) {
+       *(.init.trampoline)
+  } PHDR(text)
+
 #ifndef EFI
   /*
    * With --orphan-sections=warn (or =error) we need to handle certain linker
-- 
2.34.1