KVM: Avoid re-reading kvm->max_halt_poll_ns during halt-polling

Avoid re-reading kvm->max_halt_poll_ns multiple times during
halt-polling except when it is explicitly useful, e.g. to check if the
max time changed across a halt. kvm->max_halt_poll_ns can be changed at
any time by userspace via KVM_CAP_HALT_POLL.

This bug is unlikely to cause any serious side-effects. In the worst
case one halt polls for shorter or longer than it should, and then is
fixed up on the next halt. Furthmore, this is still possible since
kvm->max_halt_poll_ns are not synchronized with halts.

Fixes: acd05785e48c ("kvm: add capability for halt polling")
Signed-off-by: David Matlack <dmatlack@google.com>
Message-Id: <20221117001657.1067231-3-dmatlack@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2a7285c..032d2fa 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3480,6 +3480,11 @@ static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
        }
 }
 
+static unsigned int kvm_vcpu_max_halt_poll_ns(struct kvm_vcpu *vcpu)
+{
+       return READ_ONCE(vcpu->kvm->max_halt_poll_ns);
+}
+
 /*
  * Emulate a vCPU halt condition, e.g. HLT on x86, WFI on arm, etc...  If halt
  * polling is enabled, busy wait for a short time before blocking to avoid the
@@ -3488,14 +3493,15 @@ static inline void update_halt_poll_stats(struct kvm_vcpu *vcpu, ktime_t start,
  */
 void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
 {
+       unsigned int max_halt_poll_ns = kvm_vcpu_max_halt_poll_ns(vcpu);
        bool halt_poll_allowed = !kvm_arch_no_poll(vcpu);
        ktime_t start, cur, poll_end;
        bool waited = false;
        bool do_halt_poll;
        u64 halt_ns;
 
-       if (vcpu->halt_poll_ns > vcpu->kvm->max_halt_poll_ns)
-               vcpu->halt_poll_ns = vcpu->kvm->max_halt_poll_ns;
+       if (vcpu->halt_poll_ns > max_halt_poll_ns)
+               vcpu->halt_poll_ns = max_halt_poll_ns;
 
        do_halt_poll = halt_poll_allowed && vcpu->halt_poll_ns;
 
@@ -3537,18 +3543,21 @@ out:
                update_halt_poll_stats(vcpu, start, poll_end, !waited);
 
        if (halt_poll_allowed) {
+               /* Recompute the max halt poll time in case it changed. */
+               max_halt_poll_ns = kvm_vcpu_max_halt_poll_ns(vcpu);
+
                if (!vcpu_valid_wakeup(vcpu)) {
                        shrink_halt_poll_ns(vcpu);
-               } else if (vcpu->kvm->max_halt_poll_ns) {
+               } else if (max_halt_poll_ns) {
                        if (halt_ns <= vcpu->halt_poll_ns)
                                ;
                        /* we had a long block, shrink polling */
                        else if (vcpu->halt_poll_ns &&
-                                halt_ns > vcpu->kvm->max_halt_poll_ns)
+                                halt_ns > max_halt_poll_ns)
                                shrink_halt_poll_ns(vcpu);
                        /* we had a short halt and our poll time is too small */
-                       else if (vcpu->halt_poll_ns < vcpu->kvm->max_halt_poll_ns &&
-                                halt_ns < vcpu->kvm->max_halt_poll_ns)
+                       else if (vcpu->halt_poll_ns < max_halt_poll_ns &&
+                                halt_ns < max_halt_poll_ns)
                                grow_halt_poll_ns(vcpu);
                } else {
                        vcpu->halt_poll_ns = 0;