Rather than unconditionally accepting reads and writes while discarding
the value written, make accesses properly conditional upon CMCI being
exposed via MCG_CAP, and arrange to actually retain the value written.
Also reflect the extra LVT in LVR.

Note that this doesn't change the status quo of us never delivering any
interrupt through this LVT.

Fixes: 70173dbb9948 ("x86/HVM: fix miscellaneous aspects of x2APIC emulation")
Fixes: 8d0a20587e4e ("x86/hvm: further restrict access to x2apic MSRs")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
---
The Fixes: tags are referencing where the explicit mentioning of APIC_CMCI
in what are now guest_{rd,wr}msr_x2apic() was introduced; the mis-handling
really pre-dates that, though.

In principle the later assignment to "nr" in vlapic_do_init() could now be
dropped again. I wasn't quite sure though whether that's a good idea.
---
v2: Introduce vlapic_has_cmci(). Re-base over change to earlier patch.

--- a/xen/arch/x86/hvm/vlapic.c
+++ b/xen/arch/x86/hvm/vlapic.c
@@ -31,10 +31,19 @@
 #include <public/hvm/ioreq.h>
 #include <public/hvm/params.h>
 
-#define LVT_BIAS(reg)                   (((reg) - APIC_LVTT) >> 4)
+#include <../cpu/mcheck/x86_mca.h> /* MCG_CMCI_P */
+
+#define LVT_BIAS(reg)                   (((reg) - APIC_CMCI) >> 4)
 
 #define LVTS \
-    LVT(LVTT), LVT(LVTTHMR), LVT(LVTPC), LVT(LVT0), LVT(LVT1), LVT(LVTERR),
+    LVT(LVTT), LVT(LVTTHMR), LVT(LVTPC), LVT(LVT0), LVT(LVT1), LVT(LVTERR), \
+    LVT(CMCI),
+
+enum {
+#define LVT(which) VLAPIC_LVT_ ## which
+    LVTS
+#undef LVT
+};
 
 static const unsigned int lvt_reg[] = {
 #define LVT(which) APIC_ ## which
@@ -57,6 +66,7 @@ static const unsigned int lvt_valid[] =
 #define LVT0_VALID    LINT_MASK
 #define LVT1_VALID    LINT_MASK
 #define LVTERR_VALID  LVT_MASK
+#define CMCI_VALID    (LVT_MASK | APIC_DM_MASK)
 #define LVT(which)    [LVT_BIAS(APIC_ ## which)] = which ## _VALID
     LVTS
 #undef LVT
@@ -78,6 +88,11 @@ static const unsigned int lvt_valid[] =
 
 static void vlapic_do_init(struct vlapic *vlapic);
 
+static bool vlapic_has_cmci(const struct vlapic *vlapic)
+{
+    return GET_APIC_MAXLVT(vlapic_get_reg(vlapic, APIC_LVR)) >= VLAPIC_LVT_CMCI;
+}
+
 static int vlapic_find_highest_vector(const void *bitmap)
 {
     const uint32_t *word = bitmap;
@@ -697,8 +712,17 @@ int guest_rdmsr_x2apic(const struct vcpu
         return X86EMUL_EXCEPTION;
 
     offset = reg << 4;
-    if ( offset == APIC_ICR )
+    switch ( offset )
+    {
+    case APIC_ICR:
         high = (uint64_t)vlapic_read_aligned(vlapic, APIC_ICR2) << 32;
+        break;
+
+    case APIC_CMCI:
+        if ( !vlapic_has_cmci(vlapic) )
+            return X86EMUL_EXCEPTION;
+        break;
+    }
 
     *val = high | vlapic_read_aligned(vlapic, offset);
 
@@ -868,6 +892,10 @@ void vlapic_reg_write(struct vcpu *v, un
         vlapic_set_reg(vlapic, APIC_ICR2, val & 0xff000000U);
         break;
 
+    case APIC_CMCI:         /* LVT CMCI */
+        if ( !vlapic_has_cmci(vlapic) )
+            break;
+        fallthrough;
     case APIC_LVTT:         /* LVT Timer Reg */
         if ( vlapic_lvtt_tdt(vlapic) !=
              ((val & APIC_TIMER_MODE_MASK) == APIC_TIMER_MODE_TSC_DEADLINE) )
@@ -1024,9 +1052,12 @@ int guest_wrmsr_x2apic(struct vcpu *v, u
             return X86EMUL_EXCEPTION;
         break;
 
+    case APIC_CMCI:
+        if ( !vlapic_has_cmci(vlapic) )
+            return X86EMUL_EXCEPTION;
+        fallthrough;
     case APIC_LVTTHMR:
     case APIC_LVTPC:
-    case APIC_CMCI:
         if ( val & ~(LVT_MASK | APIC_DM_MASK) )
             return X86EMUL_EXCEPTION;
         break;
@@ -1438,9 +1469,11 @@ static void vlapic_do_init(struct vlapic
     if ( !has_vlapic(vlapic_vcpu(vlapic)->domain) )
         return;
 
+    nr = ARRAY_SIZE(lvt_reg) -
+         !(vlapic_vcpu(vlapic)->arch.vmce.mcg_cap & MCG_CMCI_P);
     vlapic_set_reg(vlapic, APIC_LVR,
                    MASK_INSR(0x14, APIC_LVR_VERSION_MASK) |
-                   MASK_INSR(ARRAY_SIZE(lvt_reg) - 1, APIC_LVR_MAXLVT_MASK));
+                   MASK_INSR(nr - 1, APIC_LVR_MAXLVT_MASK));
 
     for ( i = 0; i < 8; i++ )
     {