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>
(cherry picked from commit 94df6e8588e35cc2028ccb3fd2921c6e6360605e)

Gbp-Pq: Name x86spec_ctrl-elide-msr_spec_ctrl-handlin.patch

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 015a9e27820710ffc613c28e13e8954dafc7d2e4..55ef79f9d02a8af7818d40762881f966cbc5a241 100644 (file)
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -327,6 +327,10 @@ 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);
+
     print_details(thunk, caps);
 }
 

diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 846b5c3c1834fcee983efde3a65b5fab34801506..a043dd6cf3175766ddcae870f50f34725adb7dff 100644 (file)
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -29,6 +29,7 @@ XEN_CPUFEATURE(SC_MSR,          (FSCAPINTS+0)*32+18) /* MSR_SPEC_CTRL used by Xe
 XEN_CPUFEATURE(SC_RSB_PV,       (FSCAPINTS+0)*32+20) /* RSB overwrite needed for PV */
 XEN_CPUFEATURE(SC_RSB_HVM,      (FSCAPINTS+0)*32+21) /* RSB overwrite needed for HVM */
 XEN_CPUFEATURE(NO_XPTI,         (FSCAPINTS+0)*32+22) /* XPTI mitigation not in use */
+XEN_CPUFEATURE(SC_MSR_IDLE,     (FSCAPINTS+0)*32+23) /* SC_MSR && default_xen_spec_ctrl */
 
 #define NCAPINTS (FSCAPINTS + 1) /* N 32-bit words worth of info */
 

diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index 7d7c42e3a6520abfa64a6d31c3f9691aa70c1b57..77f92ba586a88241a904c694714592a1fe7d03c5 100644 (file)
--- 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_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" );
 }
 
@@ -67,7 +67,7 @@ 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" );
 }