x86: don't allow clearing of TF_kernel_mode for other than 64-bit PV

The flag is really only meant for those, both HVM and 32-bit PV tell
kernel from user mode based on CPL/RPL. Remove the all-question-marks
comment and let's be on the safe side here and also suppress clearing
for 32-bit PV (this isn't a fast path after all).

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/domain.c
+++ b/xen/arch/x86/domain.c
@@ -950,9 +950,15 @@ int arch_set_info_guest(
 
     v->fpu_initialised = !!(flags & VGCF_I387_VALID);
 
-    v->arch.flags &= ~TF_kernel_mode;
-    if ( (flags & VGCF_in_kernel) || is_hvm_domain(d)/*???*/ )
-        v->arch.flags |= TF_kernel_mode;
+    v->arch.flags |= TF_kernel_mode;
+    if ( unlikely(!(flags & VGCF_in_kernel)) &&
+         /*
+          * TF_kernel_mode is only allowed to be clear for 64-bit PV. See
+          * update_cr3(), sh_update_cr3(), and shadow_one_bit_disable() for
+          * why that is.
+          */
+         !is_hvm_domain(d) && !is_pv_32bit_domain(d) )
+        v->arch.flags &= ~TF_kernel_mode;
 
     v->arch.vgc_flags = flags;