[Xen-devel] [PATCH v3 1/5] x86/boot: Only jump into low trampoline code for real-mode boot

[David Woodhouse <dwmw2@infradead.org>]

From: David Woodhouse <dwmw@amazon.co.uk>

If the no-real-mode flag is set, don't go there at all. This is a prelude
to not even putting it there in the first place.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 xen/arch/x86/boot/head.S       | 10 ++++++++++
 xen/arch/x86/boot/trampoline.S |  4 ----
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/xen/arch/x86/boot/head.S b/xen/arch/x86/boot/head.S
index 26b680521d..d303379083 100644
--- a/xen/arch/x86/boot/head.S
+++ b/xen/arch/x86/boot/head.S
@@ -727,7 +727,17 @@ trampoline_setup:
         /* Switch to low-memory stack which lives at the end of trampoline region. */
         mov     sym_fs(trampoline_phys),%edi
         lea     TRAMPOLINE_SPACE+TRAMPOLINE_STACK_SPACE(%edi),%esp
+        cmpb    $0, sym_fs(skip_realmode)
+        jz      1f
+        /* If no-real-mode, jump straight to trampoline_protmode_entry */
+        lea     trampoline_protmode_entry-trampoline_start(%edi),%eax
+        /* EBX == 0 indicates we are the BP (Boot Processor). */
+        xor     %ebx,%ebx
+        jmp     2f
+1:
+        /* Go via 16-bit code in trampoline_boot_cpu_entry */
         lea     trampoline_boot_cpu_entry-trampoline_start(%edi),%eax
+2:
         pushl   $BOOT_CS32
         push    %eax
 
diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index 7c6a2328d2..429a088b19 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -194,9 +194,6 @@ gdt_48: .word   6*8-1
 
         .code32
 trampoline_boot_cpu_entry:
-        cmpb    $0,bootsym_rel(skip_realmode,5)
-        jnz     .Lskip_realmode
-
         /* Load pseudo-real-mode segments. */
         mov     $BOOT_PSEUDORM_DS,%eax
         mov     %eax,%ds
@@ -276,7 +273,6 @@ trampoline_boot_cpu_entry:
         mov     %eax,%gs
         mov     %eax,%ss
 
-.Lskip_realmode:
         /* EBX == 0 indicates we are the BP (Boot Processor). */
         xor     %ebx,%ebx
 
-- 
2.21.0


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