KVM: x86: Invoke kvm_mmu_unload() directly on CR4.PCIDE change

Replace a KVM_REQ_MMU_RELOAD request with a direct kvm_mmu_unload() call
when the guest's CR4.PCIDE changes.  This will allow tweaking the logic
of KVM_REQ_MMU_RELOAD to free only obsolete/invalid roots, which is the
historical intent of KVM_REQ_MMU_RELOAD.  The recent PCIDE behavior is
the only user of KVM_REQ_MMU_RELOAD that doesn't mark affected roots as
obsolete, needs to unconditionally unload the entire MMU, _and_ affects
only the current vCPU.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20220225182248.3812651-3-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/x86.c

@@ -1079,7 +1079,7 @@ void kvm_post_set_cr4(struct kvm_vcpu *vcpu, unsigned long old_cr4, unsigned lon
 	 */
 	if (!tdp_enabled &&
 	    (cr4 & X86_CR4_PCIDE) && !(old_cr4 & X86_CR4_PCIDE))
-		kvm_make_request(KVM_REQ_MMU_RELOAD, vcpu);
+		kvm_mmu_unload(vcpu);
 
 	/*
 	 * The TLB has to be flushed for all PCIDs if any of the following