x86/spec-ctrl: Don't use spec_ctrl_{enter,exit}_idle() for S3

'idle' here refers to hlt/mwait.  The S3 path isn't an idle path - it is a
platform reset.

We need to load default_xen_spec_ctrl unilaterally on the way back up.
Currently it happens as a side effect of X86_FEATURE_SC_MSR_IDLE or the next
return-to-guest, but that's fragile behaviour.

Conversely, there is no need to clear IBRS and flush the store buffers on the
way down; we're microseconds away from cutting power.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
(cherry picked from commit 71fac402e0)

xen/arch/x86/acpi/power.c

@@ -245,7 +245,6 @@ static int enter_state(u32 state)
         error = 0;
 
     ci = get_cpu_info();
-    spec_ctrl_enter_idle(ci);
     /* Avoid NMI/#MC using MSR_SPEC_CTRL until we've reloaded microcode. */
     ci->spec_ctrl_flags &= ~SCF_ist_wrmsr;
 
@@ -295,7 +294,9 @@ static int enter_state(u32 state)
 
     /* Re-enabled default NMI/#MC use of MSR_SPEC_CTRL. */
     ci->spec_ctrl_flags |= (default_spec_ctrl_flags & SCF_ist_wrmsr);
-    spec_ctrl_exit_idle(ci);
+
+    if ( boot_cpu_has(X86_FEATURE_IBRSB) )
+        wrmsrl(MSR_SPEC_CTRL, default_xen_spec_ctrl);
 
     if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
         wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);