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

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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: acd05785e4 ("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>
This commit is contained in:
David Matlack 2022-11-16 16:16:56 -08:00 committed by Paolo Bonzini
parent 97b6847ac1
commit 175d5dc79d
1 changed files with 15 additions and 6 deletions
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21
virt/kvm/kvm_main.c
View File

@ -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;