[PATCH v2 3/4] x86/efi: avoid a relocation in efi_arch_post_exit_boot()

[Frediano Ziglio <freddy77@xxxxxxxxx>]

From: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Instead of using the absolute __start_xen address, calculate it as an
offset from the current instruction pointer.  The relocation would be
problematic if the generated PE binary had .init.text as a standalone
section with just read and execute permissions."

Removing this relocation is necessary to make it safe to split .init.

No functional change intended.

Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
--
Changes since v2:
- Improve commit message.
---
 xen/arch/x86/efi/efi-boot.h | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index d738b839ee..b983f054b5 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -270,7 +270,9 @@ static void __init noreturn efi_arch_post_exit_boot(void)
 
                    /* Jump to higher mappings. */
                    "mov    stack_start(%%rip), %%rsp\n\t"
-                   "movabs $__start_xen, %[rip]\n\t"
+                   "lea    __start_xen(%%rip), %[rip]\n\t"
+                   "add    %[offset], %[rip]\n\t"
+
                    "push   %[cs]\n\t"
                    "push   %[rip]\n\t"
                    "lretq"
@@ -278,7 +280,8 @@ static void __init noreturn efi_arch_post_exit_boot(void)
                      [cr4] "+&r" (cr4)
                    : [cr3] "r" (idle_pg_table),
                      [cs] "i" (__HYPERVISOR_CS),
-                     [ds] "r" (__HYPERVISOR_DS)
+                     [ds] "r" (__HYPERVISOR_DS),
+                     [offset] "r" (__XEN_VIRT_START - xen_phys_start)
                    : "memory" );
     unreachable();
 }
-- 
2.43.0