KVM: x86: Acquire SRCU in WRMSR fastpath iff instruction needs to be skipped

Acquire SRCU in the WRMSR fastpath if and only if an instruction needs to
be skipped, i.e. only if the fastpath succeeds.  The reasoning in commit
3f2739b ("KVM: x86: Acquire SRCU read lock when handling fastpath MSR
writes") about "avoid having to play whack-a-mole" seems sound, but in
hindsight unconditionally acquiring SRCU does more harm than good.

While acquiring/releasing SRCU isn't slow per se, the things that are
_protected_ by kvm->srcu are generally safe to access only in the "slow"
VM-Exit path.  E.g. accessing memslots in generic helpers is never safe,
because accessing guest memory with IRQs disabled is unless unsafe (except
when kvm_vcpu_read_guest_atomic() is used, but that API should never be
used in emulation helpers).

In other words, playing whack-a-mole is actually desirable in this case,
because every access to an asset protected by kvm->srcu warrants further
scrutiny.

Link: https://lore.kernel.org/r/20250805190526.1453366-7-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/x86.c

@@ -2159,10 +2159,8 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
 {
 	u32 msr = kvm_rcx_read(vcpu);
 	u64 data;
-	fastpath_t ret;
 	bool handled;
-
-	kvm_vcpu_srcu_read_lock(vcpu);
+	int r;
 
 	switch (msr) {
 	case APIC_BASE_MSR + (APIC_ICR >> 4):
@@ -2178,19 +2176,16 @@ fastpath_t handle_fastpath_set_msr_irqoff(struct kvm_vcpu *vcpu)
 		break;
 	}
 
-	if (handled) {
-		if (!kvm_skip_emulated_instruction(vcpu))
-			ret = EXIT_FASTPATH_EXIT_USERSPACE;
-		else
-			ret = EXIT_FASTPATH_REENTER_GUEST;
-		trace_kvm_msr_write(msr, data);
-	} else {
-		ret = EXIT_FASTPATH_NONE;
-	}
+	if (!handled)
+		return EXIT_FASTPATH_NONE;
 
+	kvm_vcpu_srcu_read_lock(vcpu);
+	r = kvm_skip_emulated_instruction(vcpu);
 	kvm_vcpu_srcu_read_unlock(vcpu);
 
-	return ret;
+	trace_kvm_msr_write(msr, data);
+
+	return r ? EXIT_FASTPATH_REENTER_GUEST : EXIT_FASTPATH_EXIT_USERSPACE;
 }
 EXPORT_SYMBOL_GPL(handle_fastpath_set_msr_irqoff);