Re: [RFC PATCH v2 03/11] KVM: Introduce kvm_vm_has_run_once

[Raghavendra Rao Ananta]

On Mon, Nov 22, 2021 at 8:31 AM Marc Zyngier <maz@xxxxxxxxxx> wrote:
>
> On Sat, 13 Nov 2021 01:22:26 +0000,
> Raghavendra Rao Ananta <rananta@xxxxxxxxxx> wrote:
> >
> > The upcoming patches need a way to detect if the VM, as
> > a whole, has started. Hence, unionize kvm_vcpu_has_run_once()
> > of all the vcpus of the VM and build kvm_vm_has_run_once()
> > to achieve the functionality.
> >
> > No functional change intended.
> >
> > Signed-off-by: Raghavendra Rao Ananta <rananta@xxxxxxxxxx>
> > ---
> >  include/linux/kvm_host.h |  2 ++
> >  virt/kvm/kvm_main.c      | 17 +++++++++++++++++
> >  2 files changed, 19 insertions(+)
> >
> > diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> > index b373929c71eb..102e00c0e21c 100644
> > --- a/include/linux/kvm_host.h
> > +++ b/include/linux/kvm_host.h
> > @@ -1854,4 +1854,6 @@ static inline bool kvm_vcpu_has_run_once(struct kvm_vcpu *vcpu)
> >       return vcpu->has_run_once;
> >  }
> >
> > +bool kvm_vm_has_run_once(struct kvm *kvm);
> > +
> >  #endif
> > diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> > index 1ec8a8e959b2..3d8d96e8f61d 100644
> > --- a/virt/kvm/kvm_main.c
> > +++ b/virt/kvm/kvm_main.c
> > @@ -4339,6 +4339,23 @@ static int kvm_vm_ioctl_get_stats_fd(struct kvm *kvm)
> >       return fd;
> >  }
> >
> > +bool kvm_vm_has_run_once(struct kvm *kvm)
> > +{
> > +     int i, ret = false;
> > +     struct kvm_vcpu *vcpu;
> > +
> > +     mutex_lock(&kvm->lock);
> > +
> > +     kvm_for_each_vcpu(i, vcpu, kvm) {
> > +             ret = kvm_vcpu_has_run_once(vcpu);
> > +             if (ret)
> > +                     break;
> > +     }
> > +
> > +     mutex_unlock(&kvm->lock);
> > +     return ret;
> > +}
>
> This is horribly racy. Nothing prevents a vcpu from running behind
> your back. If you want any sort of guarantee, look at what we do in
> kvm_vgic_create(). Alexandru has patches that extract it to make it
> generally available (at least for arm64).
>
Yes, I looked into kvm_lock_all_vcpus(), but the fact that the series
would call the function with the current vcpu lock held caused me to
back off..
Perhaps I can come up with a similar function, kvm_lock_all_vcpus_except(vcpu) ?

Regards,
Raghavendra

>         M.
>
> --
> Without deviation from the norm, progress is not possible.