[xen staging] x86/spec-ctrl: Only adjust MSR_SPEC_CTRL for idle with legacy IBRS

[patchbot@xxxxxxx]

commit ffc7694e0c99eea158c32aa164b7d1e1bb1dc46b
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Jun 27 11:54:27 2022 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Jun 30 18:07:13 2022 +0100

    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: 8d03080d2a33 ("x86/spec-ctrl: Cease using thunk=lfence on AMD")
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 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 bd45a144ee..493d338a08 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 751355f471..7e83e0179f 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 1f275ad1fb..57f4fcb213 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);
--
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