x86: Restore IA32_MISC_ENABLE on wakeup

Code in intel.c:early_init_intel() modifies IA32_MISC_ENABLE MSR. Those
modifications must be restored after resuming from S3 (see e.g. Linux wakeup
code), otherwise bad things may happen (e.g. wakeup code may cause #GP when
trying to set IA32_EFER.NXE [1]).

This bug was noticed on a ThinkPad x230 with NX disabled in the BIOS:
Xen could correctly boot, but crashed when resuming from suspend.
Applying this patch fixed the problem.

[1] Intel SDM vol 3: "If the execute-disable capability is not
available, a write to set IA32_EFER.NXE produces a #GP exception."

Signed-off-by: Michał Kowalczyk <mkow@invisiblethingslab.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/boot/wakeup.S

@@ -138,6 +138,21 @@ wakeup_32:
         add     bootsym_rel(trampoline_xen_phys_start,4,%eax)
         mov     %eax,%cr3
 
+        /* Reapply IA32_MISC_ENABLE modifications from early_init_intel(). */
+        mov     bootsym_rel(trampoline_misc_enable_off, 4, %esi)
+        mov     bootsym_rel(trampoline_misc_enable_off + 4, 4, %edi)
+        mov     %esi, %eax
+        or      %edi, %eax
+        jz      1f
+        mov     $MSR_IA32_MISC_ENABLE, %ecx
+        rdmsr
+        not     %esi
+        not     %edi
+        and     %esi, %eax
+        and     %edi, %edx
+        wrmsr
+1:
+
         /* Will cpuid feature change after resume? */
         /* Set up EFER (Extended Feature Enable Register). */
         mov     bootsym_rel(cpuid_ext_features,4,%edi)