KVM: x86/mmu: Disable MMIO caching if MMIO value collides with L1TF

Disable MMIO caching if the MMIO value collides with the L1TF mitigation
that usurps high PFN bits.  In practice this should never happen as only
CPUs with SME support can generate such a collision (because the MMIO
value can theoretically get adjusted into legal memory), and no CPUs
exist that support SME and are susceptible to L1TF.  But, closing the
hole is trivial.
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20210225204749.1512652-5-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/spte.c

@@ -245,8 +245,19 @@ u64 mark_spte_for_access_track(u64 spte)
 void kvm_mmu_set_mmio_spte_mask(u64 mmio_value, u64 access_mask)
 {
 	BUG_ON((u64)(unsigned)access_mask != access_mask);
-	WARN_ON(mmio_value & (shadow_nonpresent_or_rsvd_mask << SHADOW_NONPRESENT_OR_RSVD_MASK_LEN));
 	WARN_ON(mmio_value & shadow_nonpresent_or_rsvd_lower_gfn_mask);
+
+	/*
+	 * Disable MMIO caching if the MMIO value collides with the bits that
+	 * are used to hold the relocated GFN when the L1TF mitigation is
+	 * enabled.  This should never fire as there is no known hardware that
+	 * can trigger this condition, e.g. SME/SEV CPUs that require a custom
+	 * MMIO value are not susceptible to L1TF.
+	 */
+	if (WARN_ON(mmio_value & (shadow_nonpresent_or_rsvd_mask <<
+				  SHADOW_NONPRESENT_OR_RSVD_MASK_LEN)))
+		mmio_value = 0;
+
 	shadow_mmio_value = mmio_value | SPTE_MMIO_MASK;
 	shadow_mmio_access_mask = access_mask;
 }