[Xen-devel] [PATCH] x86/boot: Drop all use of lmsw

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

lmsw is an obsolete relic of the 286 processor - so much so that it even lacks
intercept assistance on AMD processors.

Use a plain mov to %cr0 which is easier to follow, certainly faster to
virtualise on AMD hardware, and almost certainly a faster microcode path in
real hardware.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 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 1b11b4757a..89f841331d 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -54,9 +54,10 @@ GLOBAL(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     $X86_CR0_PE, %eax
+        mov     %eax, %cr0
+
         ljmpl   $BOOT_CS32,$bootsym_rel(trampoline_protmode_entry,6)
 
         .code32
@@ -252,9 +253,8 @@ 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     $X86_CR0_PE, %eax
+        mov     %eax, %cr0
 
         /* 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 e3cb9e033a..2af6c6017c 100644
--- a/xen/arch/x86/boot/wakeup.S
+++ b/xen/arch/x86/boot/wakeup.S
@@ -51,8 +51,9 @@ ENTRY(wakeup_start)
         lidt    wakesym(idt_48)
         lgdt    wakesym(gdt_48)
 
-        movw    $1, %ax
-        lmsw    %ax             # Turn on CR0.PE 
+        mov     $X86_CR0_PE, %eax
+        mov     %eax, %cr0
+
         ljmpl   $BOOT_CS32, $bootsym_rel(wakeup_32, 6)
 
 /* This code uses an extended set of video mode numbers. These include:
-- 
2.11.0


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