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

[patchbot@xxxxxxx]

commit 94df6e8588e35cc2028ccb3fd2921c6e6360605e
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon May 7 14:06:16 2018 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Wed May 16 12:19:10 2018 +0100

    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>
    Release-acked-by: Juergen Gross <jgross@xxxxxxxx>
---
 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 f489f79cc6..c7ec9a34d5 100644
--- 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 f9aa5d7590..32b7f0457e 100644
--- 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 86a3dfe964..9a308241c9 100644
--- 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" );
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#master

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