File 5afc13ae-6-x86-elide-MSR_SPEC_CTRL-handling-in-idle.patch of Package xen.7985

# Commit 94df6e8588e35cc2028ccb3fd2921c6e6360605e
# Date 2018-05-16 12:19:10 +0100
# Author Andrew Cooper <andrew.cooper3@citrix.com>
# Committer Andrew Cooper <andrew.cooper3@citrix.com>
x86/spec_ctrl: Elide MSR_SPEC_CTRL handling in idle context when possible

If Xen is virtualising MSR_SPEC_CTRL handling for guests, but using 0 as its
own MSR_SPEC_CTRL value, spec_ctrl_{enter,exit}_idle() need not write to the
MSR.

Requested-by: Jan Beulich <JBeulich@suse.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -327,6 +327,10 @@ void __init init_speculation_mitigations
     /* (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 )
+        setup_force_cpu_cap(X86_FEATURE_SC_MSR_IDLE);
+
     print_details(thunk, caps);
 }
 
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -30,3 +30,4 @@ XEN_CPUFEATURE(SC_MSR,          (FSCAPIN
 XEN_CPUFEATURE(SC_RSB_PV,       (FSCAPINTS+0)*32+18) /* RSB overwrite needed for PV */
 XEN_CPUFEATURE(SC_RSB_HVM,      (FSCAPINTS+0)*32+19) /* RSB overwrite needed for HVM */
 XEN_CPUFEATURE(NO_XPTI,         (FSCAPINTS+0)*32+20) /* XPTI mitigation not in use */
+XEN_CPUFEATURE(SC_MSR_IDLE,     (FSCAPINTS+0)*32+21) /* SC_MSR && default_xen_spec_ctrl */
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -52,7 +52,7 @@ static always_inline void spec_ctrl_ente
     barrier();
     info->spec_ctrl_flags |= SCF_use_shadow;
     barrier();
-    asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_SC_MSR)
+    asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_SC_MSR_IDLE)
                    :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0) : "memory" );
 }
 
@@ -67,7 +67,7 @@ static always_inline void spec_ctrl_exit
      */
     info->spec_ctrl_flags &= ~SCF_use_shadow;
     barrier();
-    asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_SC_MSR)
+    asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_SC_MSR_IDLE)
                    :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0) : "memory" );
 }