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SubjectRe: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST_STOPPED
On 01/30/2012 05:07 PM, Avi Kivity wrote:
> > +Parameters: None
> > +Returns: 0 on success, -1 on error
> > +
> > +This signals to the host kernel that the specified guest is being paused by
> > +userspace. The host will set a flag in the pvclock structure that is checked
> > +from the soft lockup watchdog. This ioctl can be called during pause or
> > +unpause.
> > +
> > 5. The kvm_run structure
> >
> >
> > +/*
> > + * kvm_set_guest_paused() indicates to the guest kernel that it has been
> > + * stopped by the hypervisor. This function will be called from the host only.
> > + */
> > +static int kvm_set_guest_paused(struct kvm *kvm)
> > +{
> > + struct kvm_vcpu *vcpu;
> > + struct pvclock_vcpu_time_info *src;
> > + int i;
> > +
> > + kvm_for_each_vcpu(i, vcpu, kvm) {
> > + if (!vcpu->arch.time_page)
> > + continue;
> > + src = &vcpu->arch.hv_clock;
> > + src->flags |= PVCLOCK_GUEST_STOPPED;
>
> This looks racy. The vcpu can remove its kvmclock concurrently with
> this access, and src will be NULL.
>
> Can you point me to the discussion that moved this to be a vm ioctl? In
> general vm ioctls that do things for all vcpus are racy, like here.
> You're accessing variables that are protected by the vcpu mutex, and not
> taking the mutex (nor can you, since it is held while the guest is
> running, unlike most kernel mutexes).
>

Note, the standard way to fix this race is to
kvm_make_request(KVM_REQ_KVMCLOCK_STOP) and kvm_vcpu_kick(vcpu), and do
the update in vcpu_enter_guest(). But I think this is needless
complexity and want to understand what motivated the move to a vm ioctl.

--
error compiling committee.c: too many arguments to function



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