[Xen-changelog] [xen stable-4.6] x86/spec_ctrl: Elide MSR_SPEC_CTRL handling in idle context when possible

[patchbot@xxxxxxx]

commit ac659af95503fd0efc442aedad37187247bbddb6
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue May 29 11:00:04 2018 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue May 29 11:00:04 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/cpu/common.c        | 3 +++
 xen/arch/x86/spec_ctrl.c         | 4 ++++
 xen/include/asm-x86/cpufeature.h | 1 +
 xen/include/asm-x86/spec_ctrl.h  | 4 ++--
 4 files changed, 10 insertions(+), 2 deletions(-)

diff --git a/xen/arch/x86/cpu/common.c b/xen/arch/x86/cpu/common.c
index 1ba1622e72..15e831abde 100644
--- a/xen/arch/x86/cpu/common.c
+++ b/xen/arch/x86/cpu/common.c
@@ -368,6 +368,9 @@ void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
                if (test_bit(X86_FEATURE_SC_RSB_HVM,
                             boot_cpu_data.x86_capability))
                        __set_bit(X86_FEATURE_SC_RSB_HVM, c->x86_capability);
+               if (test_bit(X86_FEATURE_SC_MSR_IDLE,
+                            boot_cpu_data.x86_capability))
+                       __set_bit(X86_FEATURE_SC_MSR_IDLE, c->x86_capability);
 
                /* AND the already accumulated flags with these */
                for ( i = 0 ; i < NCAPINTS ; i++ )
diff --git a/xen/arch/x86/spec_ctrl.c b/xen/arch/x86/spec_ctrl.c
index 91e18487f0..778f8e5833 100644
--- a/xen/arch/x86/spec_ctrl.c
+++ b/xen/arch/x86/spec_ctrl.c
@@ -324,6 +324,10 @@ 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 )
+        __set_bit(X86_FEATURE_SC_MSR_IDLE, boot_cpu_data.x86_capability);
+
     print_details(thunk, caps);
 }
 
diff --git a/xen/include/asm-x86/cpufeature.h b/xen/include/asm-x86/cpufeature.h
index 9c8bca9faa..d140982708 100644
--- a/xen/include/asm-x86/cpufeature.h
+++ b/xen/include/asm-x86/cpufeature.h
@@ -76,6 +76,7 @@
 #define X86_FEATURE_XTOPOLOGY    (3*32+13) /* cpu topology enum extensions */
 #define X86_FEATURE_CPUID_FAULTING (3*32+14) /* cpuid faulting */
 #define X86_FEATURE_CLFLUSH_MONITOR (3*32+15) /* clflush reqd with monitor */
+#define X86_FEATURE_SC_MSR_IDLE     (3*32+16) /* SC_MSR && 
default_xen_spec_ctrl */
 
 /* Intel-defined CPU features, CPUID level 0x00000001 (ecx), word 4 */
 #define X86_FEATURE_XMM3       (4*32+ 0) /* Streaming SIMD Extensions-3 */
diff --git a/xen/include/asm-x86/spec_ctrl.h b/xen/include/asm-x86/spec_ctrl.h
index 6c11562f71..ec943e18e3 100644
--- a/xen/include/asm-x86/spec_ctrl.h
+++ b/xen/include/asm-x86/spec_ctrl.h
@@ -54,7 +54,7 @@ static always_inline void spec_ctrl_enter_idle(struct 
cpu_info *info)
     barrier();
     asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", %c3)
                    :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0),
-                      "i" (X86_FEATURE_SC_MSR)
+                      "i" (X86_FEATURE_SC_MSR_IDLE)
                    : "memory" );
 }
 
@@ -71,7 +71,7 @@ static always_inline void spec_ctrl_exit_idle(struct cpu_info 
*info)
     barrier();
     asm volatile ( ALTERNATIVE(ASM_NOP3, "wrmsr", %c3)
                    :: "a" (val), "c" (MSR_SPEC_CTRL), "d" (0),
-                      "i" (X86_FEATURE_SC_MSR)
+                      "i" (X86_FEATURE_SC_MSR_IDLE)
                    : "memory" );
 }
 
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.6

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