Commit 175d5dc7 authored by David Matlack's avatar David Matlack Committed by Paolo Bonzini
Browse files

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: acd05785 ("kvm: add capability for halt polling")
Signed-off-by: default avatarDavid Matlack <dmatlack@google.com>
Message-Id: <20221117001657.1067231-3-dmatlack@google.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 97b6847a

virt/kvm/kvm_main.c

+15 −6
Original line number Diff line number Diff line
@@ -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 @@ void kvm_vcpu_halt(struct kvm_vcpu *vcpu) 
		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;