[Xen-changelog] [xen stable-4.9] x86/spec_ctrl: Elide MSR_SPEC_CTRL handling in idle context when possible

[patchbot@xxxxxxx]

commit c2029b462eefe0ae53088c19074b4f7f1d44e72c
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue May 29 09:46:31 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 29 09:46:31 2018 +0200

    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@xxxxxxxx>
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Wei Liu <wei.liu2@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    master commit: 94df6e8588e35cc2028ccb3fd2921c6e6360605e
    master date: 2018-05-16 12:19:10 +0100
---
 xen/arch/x86/spec_ctrl.c          | 4 ++++
 xen/include/asm-x86/cpufeatures.h | 1 +
 xen/include/asm-x86/spec_ctrl.h   | 4 ++--
 3 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 015a9e2782..55ef79f9d0 100644
--- 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/cpufeatures.h 
b/xen/include/asm-x86/cpufeatures.h
index 1353fe5046..f419c3669b 100644
--- a/xen/include/asm-x86/cpufeatures.h
+++ b/xen/include/asm-x86/cpufeatures.h
@@ -32,3 +32,4 @@ 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 */
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index 7d7c42e3a6..77f92ba586 100644
--- 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" );
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.9

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