[PATCH 06/18] KVM: x86: Acquire SRCU in WRMSR fastpath iff instruction needs to be skipped

[Sean Christopherson <seanjc@google.com>]

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
3f2739bd1e0b ("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.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
 arch/x86/kvm/x86.c | 21 ++++++++-------------
 1 file changed, 8 insertions(+), 13 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 63ca9185d133..69c668f4d2b6 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2158,10 +2158,8 @@ 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):
@@ -2177,19 +2175,16 @@ 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);
 
-- 
2.50.1.565.gc32cd1483b-goog