x86/kvm/mmu: reset MMU context when 32-bit guest switches PAE

Commit 47c42e6b ("KVM: x86: fix handling of role.cr4_pae and rename it
to 'gpte_size'") introduced a regression: 32-bit PAE guests stopped
working. The issue appears to be: when guest switches (enables) PAE we need
to re-initialize MMU context (set context->root_level, do
reset_rsvds_bits_mask(), ...) but init_kvm_tdp_mmu() doesn't do that
because we threw away is_pae(vcpu) flag from mmu role. Restore it to
kvm_mmu_extended_role (as we now don't need it in base role) to fix
the issue.

Fixes: 47c42e6b ("KVM: x86: fix handling of role.cr4_pae and rename it to 'gpte_size'")
Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/include/asm/kvm_host.h

@@ -295,6 +295,7 @@ union kvm_mmu_extended_role {
 		unsigned int valid:1;
 		unsigned int execonly:1;
 		unsigned int cr0_pg:1;
+		unsigned int cr4_pae:1;
 		unsigned int cr4_pse:1;
 		unsigned int cr4_pke:1;
 		unsigned int cr4_smap:1;

arch/x86/kvm/mmu.c

@@ -4781,6 +4781,7 @@ static union kvm_mmu_extended_role kvm_calc_mmu_role_ext(struct kvm_vcpu *vcpu)
 	union kvm_mmu_extended_role ext = {0};
 
 	ext.cr0_pg = !!is_paging(vcpu);
+	ext.cr4_pae = !!is_pae(vcpu);
 	ext.cr4_smep = !!kvm_read_cr4_bits(vcpu, X86_CR4_SMEP);
 	ext.cr4_smap = !!kvm_read_cr4_bits(vcpu, X86_CR4_SMAP);
 	ext.cr4_pse = !!is_pse(vcpu);