KVM: x86: Fold WRMSR fastpath helpers into the main handler

Fold the per-MSR WRMSR fastpath helpers into the main handler now that the
IPI path in particular is relatively tiny.  In addition to eliminating a
decent amount of boilerplate, this removes the ugly -errno/1/0 => bool
conversion (which is "necessitated" by kvm_x2apic_icr_write_fast()).

Opportunistically drop the comment about IPIs, as the purpose of the
fastpath is hopefully self-evident, and _if_ it needs more documentation,
the documentation (and rules!) should be placed in a more central location.

No functional change intended.

Link: https://lore.kernel.org/r/20250805190526.1453366-9-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -2134,48 +2134,24 @@ static inline bool kvm_vcpu_exit_request(struct kvm_vcpu *vcpu)
 	       kvm_request_pending(vcpu) || xfer_to_guest_mode_work_pending();
 }
 
-/*
- * The fast path for frequent and performance sensitive wrmsr emulation,
- * i.e. the sending of IPI, sending IPI early in the VM-Exit flow reduces
- * the latency of virtual IPI by avoiding the expensive bits of transitioning
- * from guest to host, e.g. reacquiring KVM's SRCU lock. In contrast to the
- * other cases which must be called after interrupts are enabled on the host.
- */
-static int handle_fastpath_set_x2apic_icr_irqoff(struct kvm_vcpu *vcpu, u64 data)
-{
-	if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic))
-		return 1;
-
-	return kvm_x2apic_icr_write_fast(vcpu->arch.apic, data);
-}
-
-static int handle_fastpath_set_tscdeadline(struct kvm_vcpu *vcpu, u64 data)
-{
-	kvm_set_lapic_tscdeadline_msr(vcpu, data);
-	return 0;
-}
-
 fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
 {
 	u64 data = kvm_read_edx_eax(vcpu);
 	u32 msr = kvm_rcx_read(vcpu);
-	bool handled;
 	int r;
 
 	switch (msr) {
 	case APIC_BASE_MSR + (APIC_ICR >> 4):
-		handled = !handle_fastpath_set_x2apic_icr_irqoff(vcpu, data);
+		if (!lapic_in_kernel(vcpu) || !apic_x2apic_mode(vcpu->arch.apic) ||
+		    kvm_x2apic_icr_write_fast(vcpu->arch.apic, data))
+			return EXIT_FASTPATH_NONE;
 		break;
 	case MSR_IA32_TSC_DEADLINE:
-		handled = !handle_fastpath_set_tscdeadline(vcpu, data);
+		kvm_set_lapic_tscdeadline_msr(vcpu, data);
 		break;
 	default:
-		handled = false;
-		break;
-	}
-
-	if (!handled)
 		return EXIT_FASTPATH_NONE;
+	}
 
 	kvm_vcpu_srcu_read_lock(vcpu);
 	r = kvm_skip_emulated_instruction(vcpu);