x86/spec-ctrl: Only adjust MSR_SPEC_CTRL for idle with legacy IBRS

Back at the time of the original Spectre-v2 fixes, it was recommended to clear
MSR_SPEC_CTRL when going idle.  This is because of the side effects on the
sibling thread caused by the microcode IBRS and STIBP implementations which
were retrofitted to existing CPUs.

However, there are no relevant cross-thread impacts for the hardware
IBRS/STIBP implementations, so this logic should not be used on Intel CPUs
supporting eIBRS, or any AMD CPUs; doing so only adds unnecessary latency to
the idle path.

Furthermore, there's no point playing with MSR_SPEC_CTRL in the idle paths if
SMT is disabled for other reasons.

Fixes: 8d03080d2a ("x86/spec-ctrl: Cease using thunk=lfence on AMD")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
(cherry picked from commit ffc7694e0c)

xen/arch/x86/spec_ctrl.c

@@ -1104,8 +1104,14 @@ void __init init_speculation_mitigations(void)
     /* (Re)init BSP state now that default_spec_ctrl_flags has been calculated. */
     init_shadow_spec_ctrl_state();
 
-    /* If Xen is using any MSR_SPEC_CTRL settings, adjust the idle path. */
-    if ( default_xen_spec_ctrl )
+    /*
+     * For microcoded IBRS only (i.e. Intel, pre eIBRS), it is recommended to
+     * clear MSR_SPEC_CTRL before going idle, to avoid impacting sibling
+     * threads.  Activate this if SMT is enabled, and Xen is using a non-zero
+     * MSR_SPEC_CTRL setting.
+     */
+    if ( boot_cpu_has(X86_FEATURE_IBRSB) && !(caps & ARCH_CAPS_IBRS_ALL) &&
+         hw_smt_enabled && default_xen_spec_ctrl )
         setup_force_cpu_cap(X86_FEATURE_SC_MSR_IDLE);
 
     xpti_init_default(caps);

xen/include/asm-x86/cpufeatures.h

@@ -33,7 +33,7 @@ XEN_CPUFEATURE(SC_MSR_HVM,        X86_SYNTH(17)) /* MSR_SPEC_CTRL used by Xen fo
 XEN_CPUFEATURE(SC_RSB_PV,         X86_SYNTH(18)) /* RSB overwrite needed for PV */
 XEN_CPUFEATURE(SC_RSB_HVM,        X86_SYNTH(19)) /* RSB overwrite needed for HVM */
 XEN_CPUFEATURE(XEN_SELFSNOOP,     X86_SYNTH(20)) /* SELFSNOOP gets used by Xen itself */
-XEN_CPUFEATURE(SC_MSR_IDLE,       X86_SYNTH(21)) /* (SC_MSR_PV || SC_MSR_HVM) && default_xen_spec_ctrl */
+XEN_CPUFEATURE(SC_MSR_IDLE,       X86_SYNTH(21)) /* Clear MSR_SPEC_CTRL on idle */
 XEN_CPUFEATURE(XEN_LBR,           X86_SYNTH(22)) /* Xen uses MSR_DEBUGCTL.LBR */
 /* Bits 23,24 unused. */
 XEN_CPUFEATURE(SC_VERW_IDLE,      X86_SYNTH(25)) /* VERW used by Xen for idle */

xen/include/asm-x86/spec_ctrl.h

@@ -78,7 +78,8 @@ static always_inline void spec_ctrl_enter_idle(struct cpu_info *info)
     uint32_t val = 0;
 
     /*
-     * Branch Target Injection:
+     * It is recommended in some cases to clear MSR_SPEC_CTRL when going idle,
+     * to avoid impacting sibling threads.
      *
      * Latch the new shadow value, then enable shadowing, then update the MSR.
      * There are no SMP issues here; only local processor ordering concerns.
@@ -114,7 +115,7 @@ static always_inline void spec_ctrl_exit_idle(struct cpu_info *info)
     uint32_t val = info->xen_spec_ctrl;
 
     /*
-     * Branch Target Injection:
+     * Restore MSR_SPEC_CTRL on exit from idle.
      *
      * Disable shadowing before updating the MSR.  There are no SMP issues
      * here; only local processor ordering concerns.