[PATCH] x86/boot: Untangle the trampoline copying/entry logic

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

The LRET is detached from the PUSHes which set it up, and this is about to get
worse with the changes to trampoline relocation.  For the sake of one variable
read, the complexity is not worth it.

Reorder the logic to copy the trampoline into place, then switch stack and
enter the trampoline.

No functional change.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Roger: I'd like this to be a prerequisite to your "[PATCH 4/7] x86/boot: apply
trampoline relocations at destination position" to avoid the movement of
reloc_trampoline32() making things worse.
---
 xen/arch/x86/boot/head.S | 17 +++++++++--------
 1 file changed, 9 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 59a2b5005cf6..4082d7c6c0a0 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -697,19 +697,20 @@ trampoline_setup:
         call    cmdline_parse_early
 
 1:
-        /* Switch to low-memory stack which lives at the end of trampoline 
region. */
-        mov     sym_esi(trampoline_phys), %edi
-        lea     TRAMPOLINE_SIZE(%edi), %esp
-        lea     trampoline_boot_cpu_entry-trampoline_start(%edi),%eax
-        pushl   $BOOT_CS32
-        push    %eax
-
         /* Copy bootstrap trampoline to low memory, below 1MB. */
         lea     sym_esi(trampoline_start), %esi
+        mov     sym_esi(trampoline_phys), %edi
         mov     $((trampoline_end - trampoline_start) / 4),%ecx
         rep movsl
 
-        /* Jump into the relocated trampoline. */
+        /* Switch to low-memory stack which lives at the end of trampoline. */
+        mov     sym_esi(trampoline_phys), %edi
+        lea     TRAMPOLINE_SIZE(%edi), %esp
+
+        /* Enter the trampoline at trampoline_boot_cpu_entry(). */
+        lea     trampoline_boot_cpu_entry - trampoline_start(%edi), %eax
+        pushl   $BOOT_CS32
+        push    %eax
         lret
 
 ENTRY(trampoline_start)
-- 
2.39.5