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

[Marc Zyngier <maz@kernel.org>]

On Sat, 13 Nov 2021 01:22:26 +0000,
Raghavendra Rao Ananta <rananta@google.com> 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@google.com>
> ---
>  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).

	M.

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