[Xen-devel] [PATCH] remove CSTAR save/restore on VMX

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

CSTAR is an unused MSR in 64-bit mode on Intel's EM64T, hence there's no need
to save/restore this MSR during context switches. Mimic real hardware by
retaining the value written for eventual guest OS reads.

(Patch won't apply without the EFER adjustment patch.)

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxxxx>

Index: 2007-05-14/xen/arch/x86/hvm/vmx/vmx.c
===================================================================
--- 2007-05-14.orig/xen/arch/x86/hvm/vmx/vmx.c  2007-05-14 14:28:19.000000000 
+0200
+++ 2007-05-14/xen/arch/x86/hvm/vmx/vmx.c       2007-05-14 14:33:24.000000000 
+0200
@@ -88,8 +88,7 @@ static DEFINE_PER_CPU(struct vmx_msr_sta
 
 static u32 msr_index[VMX_MSR_COUNT] =
 {
-    MSR_LSTAR, MSR_STAR, MSR_CSTAR,
-    MSR_SYSCALL_MASK
+    MSR_LSTAR, MSR_STAR, MSR_SYSCALL_MASK
 };
 
 static void vmx_save_host_msrs(void)
@@ -147,7 +146,7 @@ static inline int long_mode_do_msr_read(
         break;
 
     case MSR_CSTAR:
-        msr_content = guest_msr_state->msrs[VMX_INDEX_MSR_CSTAR];
+        msr_content = v->arch.hvm_vmx.cstar;
         break;
 
     case MSR_SYSCALL_MASK:
@@ -250,7 +249,8 @@ static inline int long_mode_do_msr_write
     case MSR_CSTAR:
         if ( !is_canonical_address(msr_content) )
             goto uncanonical_address;
-        WRITE_MSR(CSTAR);
+        v->arch.hvm_vmx.cstar = msr_content;
+        break;
 
     case MSR_SYSCALL_MASK:
         WRITE_MSR(SYSCALL_MASK);
@@ -730,12 +730,12 @@ static void vmx_save_cpu_state(struct vc
     unsigned long guest_flags = guest_state->flags;
 
     data->shadow_gs = v->arch.hvm_vmx.shadow_gs;
+    data->msr_cstar = v->arch.hvm_vmx.cstar;
 
     /* save msrs */
     data->msr_flags        = guest_flags;
     data->msr_lstar        = guest_state->msrs[VMX_INDEX_MSR_LSTAR];
     data->msr_star         = guest_state->msrs[VMX_INDEX_MSR_STAR];
-    data->msr_cstar        = guest_state->msrs[VMX_INDEX_MSR_CSTAR];
     data->msr_syscall_mask = guest_state->msrs[VMX_INDEX_MSR_SYSCALL_MASK];
 #endif
 
@@ -755,9 +755,9 @@ static void vmx_load_cpu_state(struct vc
     guest_state->flags = data->msr_flags;
     guest_state->msrs[VMX_INDEX_MSR_LSTAR]        = data->msr_lstar;
     guest_state->msrs[VMX_INDEX_MSR_STAR]         = data->msr_star;
-    guest_state->msrs[VMX_INDEX_MSR_CSTAR]        = data->msr_cstar;
     guest_state->msrs[VMX_INDEX_MSR_SYSCALL_MASK] = data->msr_syscall_mask;
 
+    v->arch.hvm_vmx.cstar     = data->msr_cstar;
     v->arch.hvm_vmx.shadow_gs = data->shadow_gs;
 #endif
 
Index: 2007-05-14/xen/include/asm-x86/hvm/vmx/vmcs.h
===================================================================
--- 2007-05-14.orig/xen/include/asm-x86/hvm/vmx/vmcs.h  2007-05-14 
14:28:19.000000000 +0200
+++ 2007-05-14/xen/include/asm-x86/hvm/vmx/vmcs.h       2007-05-14 
14:33:24.000000000 +0200
@@ -37,7 +37,6 @@ struct vmcs_struct {
 enum {
     VMX_INDEX_MSR_LSTAR = 0,
     VMX_INDEX_MSR_STAR,
-    VMX_INDEX_MSR_CSTAR,
     VMX_INDEX_MSR_SYSCALL_MASK,
 
     VMX_MSR_COUNT
@@ -77,6 +76,7 @@ struct arch_vmx_struct {
 #ifdef __x86_64__
     struct vmx_msr_state msr_state;
     unsigned long        shadow_gs;
+    unsigned long        cstar;
 #endif
     unsigned long        efer;
     unsigned long        vmxassist_enabled:1;




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