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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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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