KVM: x86: drop superfluous mmu_check_root() from fast_pgd_switch()

The mmu_check_root() check in fast_pgd_switch() seems to be
superfluous: when GPA is outside of the visible range
cached_root_available() will fail for non-direct roots
(as we can't have a matching one on the list) and we don't
seem to care for direct ones.

Also, raising #TF immediately when a non-existent GFN is written to CR3
doesn't seem to mach architectural behavior. Drop the check.
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Message-Id: <20200710141157.1640173-10-vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/mmu.c

@@ -4243,8 +4243,7 @@ static bool fast_pgd_switch(struct kvm_vcpu *vcpu, gpa_t new_pgd,
 	 */
 	if (mmu->shadow_root_level >= PT64_ROOT_4LEVEL &&
 	    mmu->root_level >= PT64_ROOT_4LEVEL)
-		return !mmu_check_root(vcpu, new_pgd >> PAGE_SHIFT) &&
-		       cached_root_available(vcpu, new_pgd, new_role);
+		return cached_root_available(vcpu, new_pgd, new_role);
 
 	return false;
 }