KVM: MMU: retain more available bits on mmio spte

Let mmio spte only use bit62 and bit63 on upper 32 bits, then bit 52 ~ bit 61
can be used for other purposes

Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Reviewed-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx.c

@@ -4176,10 +4176,10 @@ static void ept_set_mmio_spte_mask(void)
 	/*
 	 * EPT Misconfigurations can be generated if the value of bits 2:0
 	 * of an EPT paging-structure entry is 110b (write/execute).
-	 * Also, magic bits (0xffull << 49) is set to quickly identify mmio
+	 * Also, magic bits (0x3ull << 62) is set to quickly identify mmio
 	 * spte.
 	 */
-	kvm_mmu_set_mmio_spte_mask(0xffull << 49 | 0x6ull);
+	kvm_mmu_set_mmio_spte_mask((0x3ull << 62) | 0x6ull);
 }
 
 /*

arch/x86/kvm/x86.c

@@ -5280,7 +5280,13 @@ static void kvm_set_mmio_spte_mask(void)
 	 * Set the reserved bits and the present bit of an paging-structure
 	 * entry to generate page fault with PFER.RSV = 1.
 	 */
-	mask = ((1ull << (62 - maxphyaddr + 1)) - 1) << maxphyaddr;
+	 /* Mask the reserved physical address bits. */
+	mask = ((1ull << (51 - maxphyaddr + 1)) - 1) << maxphyaddr;
+
+	/* Bit 62 is always reserved for 32bit host. */
+	mask |= 0x3ull << 62;
+
+	/* Set the present bit. */
 	mask |= 1ull;
 
 #ifdef CONFIG_X86_64