KVM: X86: Raise #GP when clearing CR0_PG in 64 bit mode

In the SDM:
If the logical processor is in 64-bit mode or if CR4.PCIDE = 1, an
attempt to clear CR0.PG causes a general-protection exception (#GP).
Software should transition to compatibility mode and clear CR4.PCIDE
before attempting to disable paging.
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
Message-Id: <20211207095230.53437-1-jiangshanlai@gmail.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -890,7 +890,8 @@ int kvm_set_cr0(struct kvm_vcpu *vcpu, unsigned long cr0)
 	    !load_pdptrs(vcpu, vcpu->arch.walk_mmu, kvm_read_cr3(vcpu)))
 		return 1;
 
-	if (!(cr0 & X86_CR0_PG) && kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE))
+	if (!(cr0 & X86_CR0_PG) &&
+	    (is_64_bit_mode(vcpu) || kvm_read_cr4_bits(vcpu, X86_CR4_PCIDE)))
 		return 1;
 
 	static_call(kvm_x86_set_cr0)(vcpu, cr0);