[Xen-changelog] [xen staging] x86/boot: Drop all use of lmsw

[patchbot@xxxxxxx]

commit f1a94202128e677be5c14f84492f80a12dba9571
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed May 1 18:14:03 2019 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed Aug 28 19:52:54 2019 +0100

    x86/boot: Drop all use of lmsw
    
    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>
    Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/boot/trampoline.S | 12 +++++-------
 xen/arch/x86/boot/wakeup.S     |  5 +++--
 2 files changed, 8 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/boot/trampoline.S b/xen/arch/x86/boot/trampoline.S
index e98db7e3cc..8dd48ece94 100644
--- a/xen/arch/x86/boot/trampoline.S
+++ b/xen/arch/x86/boot/trampoline.S
@@ -53,10 +53,9 @@ GLOBAL(trampoline_realmode_entry)
         cli
         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, %ebx         # EBX != 0 indicates we are an AP
+        mov     %ebx, %cr0                # Alias with CR0.PE for brevity
+
         ljmpl   $BOOT_CS32,$bootsym_rel(trampoline_protmode_entry,6)
 
 trampoline_gdt:
@@ -263,9 +262,8 @@ 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 090487ba78..6dbb1bc0eb 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:
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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