[PATCH v2 4/9] x86/spec-ctrl: Don't use spec_ctrl_{enter,exit}_idle() for S3

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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

Conditionally clearing IBRS and flushing the store buffers on the way down is
a waste of time.

Furthermore, we want to load default_xen_mcu_opt_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.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
CC: Wei Liu <wl@xxxxxxx>

v2:
 * New
---
 xen/arch/x86/acpi/power.c | 5 +++--
 1 file changed, 3 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/acpi/power.c b/xen/arch/x86/acpi/power.c
index 31a56f02d083..ea2bd8bbfe93 100644
--- a/xen/arch/x86/acpi/power.c
+++ b/xen/arch/x86/acpi/power.c
@@ -248,7 +248,6 @@ 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_mcu_opt_ctrl);
 
     if ( boot_cpu_has(X86_FEATURE_SRBDS_CTRL) )
         wrmsrl(MSR_MCU_OPT_CTRL, default_xen_mcu_opt_ctrl);
-- 
2.11.0