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authorDavid Matlack <dmatlack@google.com>2022-11-16 16:16:56 -0800
committerPaolo Bonzini <pbonzini@redhat.com>2022-11-17 10:49:14 -0500
commit175d5dc79dcb58d18e11192656eefa27abb3fd33 (patch)
tree06b7980324a976c97ea31b9660b5b6b57bea7b27 /virt
parent97b6847ac10ba753849207e20df74a7c0ea03343 (diff)
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>
Diffstat (limited to 'virt')
-rw-r--r--virt/kvm/kvm_main.c21
1 files changed, 15 insertions, 6 deletions
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2a7285c14013..032d2fa301f5 100644
--- 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;