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[Xen-devel] [PATCH] x86/boot: Use mov to cr0 in preference to lmsw



lmsw is for compability for 286 processors only, and any more modern
processors are recomended to use mov to cr0.  Xen has never been capable of
booting on a 286, given its multiboot entry.

Furthermore, this avoids needless playing with the CD, NW and AM bits.  These
do get explicitly chosen slightly later on boot.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <JBeulich@xxxxxxxx>
---
 xen/arch/x86/boot/trampoline.S |   12 ++++++------
 xen/arch/x86/boot/wakeup.S     |    5 +++--
 2 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index f84ce2a..89e292a 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -28,9 +28,9 @@ trampoline_realmode_entry:
         lidt    bootsym(idt_48)
         lgdt    bootsym(gdt_48)
         mov     $1,%bl                    # EBX != 0 indicates we are an AP
-        xor     %ax, %ax
-        inc     %ax
-        lmsw    %ax                       # CR0.PE = 1 (enter protected mode)
+        mov     %cr0,%eax
+        or      $X86_CR0_PE,%al
+        mov     %eax,%cr0                 # CR0.PE = 1 (enter protected mode)
         ljmpl   $BOOT_CS32,$bootsym_rel(trampoline_protmode_entry,6)
 
 idt_48: .word   0, 0, 0 # base = limit = 0
@@ -182,9 +182,9 @@ trampoline_boot_cpu_entry:
         lgdt    bootsym(gdt_48)
 
         /* Enter protected mode, and flush insn queue. */
-        xor     %ax,%ax
-        inc     %ax
-        lmsw    %ax                       # CR0.PE = 1 (enter protected mode)
+        mov     %cr0,%eax
+        or      $X86_CR0_PE,%al
+        mov     %eax,%cr0                 # CR0.PE = 1 (enter protected mode)
 
         /* Load proper protected-mode values into all segment registers. */
         ljmpl   $BOOT_CS32,$bootsym_rel(1f,6)
diff --git a/xen/arch/x86/boot/wakeup.S b/xen/arch/x86/boot/wakeup.S
index b1d4787..2b54219 100644
--- a/xen/arch/x86/boot/wakeup.S
+++ b/xen/arch/x86/boot/wakeup.S
@@ -46,8 +46,9 @@ ENTRY(wakeup_start)
         lidt    %fs:bootsym(idt_48)
         lgdt    %fs:bootsym(gdt_48)
 
-        movw    $1, %ax
-        lmsw    %ax             # Turn on CR0.PE 
+        mov     %cr0,%eax
+        or      $X86_CR0_PE,%al
+        mov     %eax,%cr0       # Turn on CR0.PE
         ljmpl   $BOOT_CS32, $bootsym_rel(wakeup_32, 6)
 
 /* This code uses an extended set of video mode numbers. These include:
-- 
1.7.10.4


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