KVM: x86: Use guest MAXPHYADDR from CPUID.0x8000_0008 iff TDP is enabled

Ignore the guest MAXPHYADDR reported by CPUID.0x8000_0008 if TDP, i.e.
NPT, is disabled, and instead use the host's MAXPHYADDR.  Per AMD'S APM:

  Maximum guest physical address size in bits. This number applies only
  to guests using nested paging. When this field is zero, refer to the
  PhysAddrSize field for the maximum guest physical address size.

Fixes: 24c82e576b78 ("KVM: Sanitize cpuid")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210623230552.4027702-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index c42613cfb5ba6d6055bf683195bd278545bfa6d0..1a4217b3e185510408afbedeff24d1fa9181be9c 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -940,8 +940,14 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
                unsigned virt_as = max((entry->eax >> 8) & 0xff, 48U);
                unsigned phys_as = entry->eax & 0xff;
 
-               if (!g_phys_as)
+               /*
+                * Use bare metal's MAXPHADDR if the CPU doesn't report guest
+                * MAXPHYADDR separately, or if TDP (NPT) is disabled, as the
+                * guest version "applies only to guests using nested paging".
+                */
+               if (!g_phys_as || !tdp_enabled)
                        g_phys_as = phys_as;
+
                entry->eax = g_phys_as | (virt_as << 8);
                entry->edx = 0;
                cpuid_entry_override(entry, CPUID_8000_0008_EBX);