x86/HVM: fix miscellaneous aspects of x2APIC emulation

- generate #GP on invalid APIC base MSR transitions
- fail reads from the self-IPI register (which is write-only)
- handle self-IPI writes and the ICR2 half of ICR writes largely in
  hvm_x2apic_msr_write()

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
---
v2: Split from main patch.

--- a/xen/arch/x86/hvm/hvm.c
+++ b/xen/arch/x86/hvm/hvm.c
@@ -4499,7 +4499,8 @@ int hvm_msr_write_intercept(unsigned int
         break;
 
     case MSR_IA32_APICBASE:
-        vlapic_msr_set(vcpu_vlapic(v), msr_content);
+        if ( !vlapic_msr_set(vcpu_vlapic(v), msr_content) )
+            goto gp_fault;
         break;
 
     case MSR_IA32_TSC_DEADLINE:
--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -602,6 +602,7 @@ int hvm_x2apic_msr_read(struct vcpu *v, 
         break;
 
     case APIC_ICR2:
+    case APIC_SELF_IPI:
         return 1;
     }
 
@@ -692,9 +693,7 @@ static int vlapic_reg_write(struct vcpu 
         break;
 
     case APIC_SELF_IPI:
-        rc = vlapic_x2apic_mode(vlapic)
-            ? vlapic_reg_write(v, APIC_ICR, 0x40000 | (val & 0xff))
-            : X86EMUL_UNHANDLEABLE;
+        rc = X86EMUL_UNHANDLEABLE;
         break;
 
     case APIC_ICR:
@@ -704,9 +703,7 @@ static int vlapic_reg_write(struct vcpu 
         break;
 
     case APIC_ICR2:
-        if ( !vlapic_x2apic_mode(vlapic) )
-            val &= 0xff000000;
-        vlapic_set_reg(vlapic, APIC_ICR2, val);
+        vlapic_set_reg(vlapic, APIC_ICR2, val & 0xff000000);
         break;
 
     case APIC_LVTT:         /* LVT Timer Reg */
@@ -865,16 +862,17 @@ int hvm_x2apic_msr_write(struct vcpu *v,
 
     switch ( offset )
     {
-        int rc;
-
     case APIC_ICR:
-        rc = vlapic_reg_write(v, APIC_ICR2, (uint32_t)(msr_content >> 32));
-        if ( rc )
-            return rc;
+        vlapic_set_reg(vlapic, APIC_ICR2, msr_content >> 32);
         break;
 
     case APIC_ICR2:
         return X86EMUL_UNHANDLEABLE;
+
+    case APIC_SELF_IPI:
+        offset = APIC_ICR;
+        msr_content = APIC_DEST_SELF | (uint8_t)msr_content;
+        break;
     }
 
     return vlapic_reg_write(v, offset, (uint32_t)msr_content);
@@ -893,10 +891,12 @@ const struct hvm_mmio_handler vlapic_mmi
     .write_handler = vlapic_write
 };
 
-void vlapic_msr_set(struct vlapic *vlapic, uint64_t value)
+bool_t vlapic_msr_set(struct vlapic *vlapic, uint64_t value)
 {
     if ( (vlapic->hw.apic_base_msr ^ value) & MSR_IA32_APICBASE_ENABLE )
     {
+        if ( unlikely(value & MSR_IA32_APICBASE_EXTD) )
+            return 0;
         if ( value & MSR_IA32_APICBASE_ENABLE )
         {
             vlapic_reset(vlapic);
@@ -905,10 +905,15 @@ void vlapic_msr_set(struct vlapic *vlapi
         }
         else
         {
+            if ( unlikely(vlapic->hw.apic_base_msr & MSR_IA32_APICBASE_EXTD) )
+                return 0;
             vlapic->hw.disabled |= VLAPIC_HW_DISABLED;
             pt_may_unmask_irq(vlapic_domain(vlapic), NULL);
         }
     }
+    else if ( !(value & MSR_IA32_APICBASE_ENABLE) &&
+              unlikely(value & MSR_IA32_APICBASE_EXTD) )
+        return 0;
 
     vlapic->hw.apic_base_msr = value;
 
@@ -923,6 +928,8 @@ void vlapic_msr_set(struct vlapic *vlapi
 
     HVM_DBG_LOG(DBG_LEVEL_VLAPIC,
                 "apic base msr is 0x%016"PRIx64, vlapic->hw.apic_base_msr);
+
+    return 1;
 }
 
 uint64_t  vlapic_tdt_msr_get(struct vlapic *vlapic)
@@ -1206,6 +1213,10 @@ static int lapic_load_hidden(struct doma
     if ( hvm_load_entry_zeroextend(LAPIC, h, &s->hw) != 0 ) 
         return -EINVAL;
 
+    if ( !(s->hw.apic_base_msr & MSR_IA32_APICBASE_ENABLE) &&
+         unlikely(s->hw.apic_base_msr & MSR_IA32_APICBASE_EXTD) )
+        return -EINVAL;
+
     vmx_vlapic_msr_changed(v);
 
     return 0;
--- a/xen/include/asm-x86/hvm/vlapic.h
+++ b/xen/include/asm-x86/hvm/vlapic.h
@@ -106,7 +106,7 @@ void vlapic_destroy(struct vcpu *v);
 
 void vlapic_reset(struct vlapic *vlapic);
 
-void vlapic_msr_set(struct vlapic *vlapic, uint64_t value);
+bool_t vlapic_msr_set(struct vlapic *vlapic, uint64_t value);
 void vlapic_tdt_msr_set(struct vlapic *vlapic, uint64_t value);
 uint64_t vlapic_tdt_msr_get(struct vlapic *vlapic);