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SubjectRe: [PATCH 3/4 V10] Add ioctl for KVMCLOCK_GUEST_STOPPED
On Mon, 30 Jan 2012, Avi Kivity wrote:

> On 01/17/2012 08:40 PM, Eric B Munson wrote:
> > Now that we have a flag that will tell the guest it was suspended, create an
> > interface for that communication using a KVM ioctl.
> >
> >
> > diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
> > index e1d94bf..1931e5c 100644
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -1491,6 +1491,19 @@ following algorithm:
> > Some guests configure the LINT1 NMI input to cause a panic, aiding in
> > debugging.
> >
> > +4.65 KVMCLOCK_GUEST_PAUSED
> > +
> > +Capability: KVM_CAP_GUEST_PAUSED
> > +Architechtures: Any that implement pvclocks (currently x86 only)
> > +Type: vcpu ioctl
>
> vm ioctl.
>
> > +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).
>

Jan Kiszka suggested that becuase there isn't a use case for notifying
individual vcpus (can vcpu's be paused individually?) that it makes more sense
to have a vm ioctl.

http://thread.gmane.org/gmane.comp.emulators.qemu/131624

If the per vcpu ioctl is the right choice I can resend those patches.

Eric
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