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>
Release-acked-by: Juergen Gross <jgross@suse.com>

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index f489f79cc6820708e6fe0e5fabb54a65d0a1b914..c7ec9a34d591ddbe842251bb26cdf64c2c6f8186 100644 (file)
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -408,6 +408,10 @@ 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 )
+        setup_force_cpu_cap(X86_FEATURE_SC_MSR_IDLE);
+
     xpti_init_default(false);
     if ( opt_xpti == 0 )
         setup_force_cpu_cap(X86_FEATURE_NO_XPTI);

diff --git a/xen/include/asm-x86/cpufeatures.h b/xen/include/asm-x86/cpufeatures.h
index f9aa5d75903b5ef7d7de4d27af4312f1308b171e..32b7f0457ecd60a46a2d48d0d740514cd9c9c76e 100644 (file)
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -30,3 +30,4 @@ XEN_CPUFEATURE(SC_MSR,          (FSCAPINTS+0)*32+16) /* MSR_SPEC_CTRL used by Xe
 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 */

diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index 86a3dfe9642c8a7276db532afcbbd93ae4ce91a2..9a308241c94ee8974c85ba7df39bb36dd7aa0a9e 100644 (file)
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -56,7 +56,7 @@ static always_inline void spec_ctrl_enter_idle(struct cpu_info *info)
     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" );
 }
 
@@ -71,7 +71,7 @@ static always_inline void spec_ctrl_exit_idle(struct cpu_info *info)
      */
     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" );
 }