[PATCH 10/10] x86/spec_ctrl: Elide MSR_SPEC_CTRL handling in idle context when possible

[Andrew Cooper <andrew.cooper3@citrix.com>]

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>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wei.liu2@citrix.com>
CC: Roger Pau Monné <roger.pau@citrix.com>
CC: Juergen Gross <jgross@suse.com>
---
 xen/arch/x86/spec_ctrl.c          | 4 ++++
 xen/include/asm-x86/cpufeatures.h | 1 +
 xen/include/asm-x86/spec_ctrl.h   | 8 ++------
 3 files changed, 7 insertions(+), 6 deletions(-)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index a2328bd..f4a3165 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -526,6 +526,10 @@ 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 9d5d81e..b90aa2d 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -31,3 +31,4 @@ XEN_CPUFEATURE(SC_MSR_HVM,      (FSCAPINTS+0)*32+17) /* 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_PV || SC_MSR_HVM) && default_xen_spec_ctrl */
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index bb4e7b2..993b958 100644
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -58,9 +58,7 @@ static always_inline void spec_ctrl_enter_idle(struct cpu_info *info)
     barrier();
     info->spec_ctrl_flags |= SCF_use_shadow;
     barrier();
-    asm volatile ( ALTERNATIVE_2(ASM_NOP3,
-                                 "wrmsr", X86_FEATURE_SC_MSR_PV,
-                                 "wrmsr", X86_FEATURE_SC_MSR_HVM)
+    asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_SC_MSR_IDLE)
                    :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0) : "memory" );
 }
 
@@ -75,9 +73,7 @@ static always_inline void spec_ctrl_exit_idle(struct cpu_info *info)
      */
     info->spec_ctrl_flags &= ~SCF_use_shadow;
     barrier();
-    asm volatile ( ALTERNATIVE_2(ASM_NOP3,
-                                 "wrmsr", X86_FEATURE_SC_MSR_PV,
-                                 "wrmsr", X86_FEATURE_SC_MSR_HVM)
+    asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", X86_FEATURE_SC_MSR_IDLE)
                    :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0) : "memory" );
 }
 
-- 
2.1.4


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