[PATCH v2 1/2] x86/spec-ctrl: Only adjust MSR_SPEC_CTRL for idle with legacy IBRS

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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: 8d03080d2a33 ("x86/spec-ctrl: Cease using thunk=lfence on AMD")
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/include/asm/cpufeatures.h |  2 +-
 xen/arch/x86/include/asm/spec_ctrl.h   |  5 +++--
 xen/arch/x86/spec_ctrl.c               | 10 ++++++++--
 3 files changed, 12 insertions(+), 5 deletions(-)

diff --git a/xen/arch/x86/include/asm/cpufeatures.h 
b/xen/arch/x86/include/asm/cpufeatures.h
index bd45a144ee78..493d338a085e 100644
--- a/xen/arch/x86/include/asm/cpufeatures.h
+++ b/xen/arch/x86/include/asm/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 
*/
diff --git a/xen/arch/x86/include/asm/spec_ctrl.h 
b/xen/arch/x86/include/asm/spec_ctrl.h
index 751355f471f4..7e83e0179fb9 100644
--- a/xen/arch/x86/include/asm/spec_ctrl.h
+++ b/xen/arch/x86/include/asm/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.
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 1f275ad1fb5d..57f4fcb21398 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -1151,8 +1151,14 @@ 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);
-- 
2.11.0