Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start

From: Sean Christopherson <[email-masked]>
Date: Sat, 8 Jan 2022 01:06:03 +0000

On Tue, Jan 04, 2022, Raghavendra Rao Ananta wrote:
> Capture the start of the KVM VM, which is basically the

Please wrap at ~75 chars.

> start of any vCPU run. This state of the VM is helpful
> in the upcoming patches to prevent user-space from
> configuring certain VM features after the VM has started
> running.

Please provide context of how the flag will be used.  I glanced at the future
patches, and knowing very little about arm, I was unable to glean useful info
about exactly who is being prevented from doing what.

> 
> Signed-off-by: Raghavendra Rao Ananta <[email-masked]>
> ---
>  include/linux/kvm_host.h | 3 +++
>  virt/kvm/kvm_main.c      | 9 +++++++++
>  2 files changed, 12 insertions(+)
> 
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index c310648cc8f1..d0bd8f7a026c 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -623,6 +623,7 @@ struct kvm {
>  	struct notifier_block pm_notifier;
>  #endif
>  	char stats_id[KVM_STATS_NAME_SIZE];
> +	bool vm_started;
>  };
>  
>  #define kvm_err(fmt, ...) \
> @@ -1666,6 +1667,8 @@ static inline bool kvm_check_request(int req, struct kvm_vcpu *vcpu)
>  	}
>  }
>  
> +#define kvm_vm_has_started(kvm) (kvm->vm_started)

Needs parantheses around (kvm), but why bother with a macro?  This is the same
header that defines struct kvm.

> +
>  extern bool kvm_rebooting;
>  
>  extern unsigned int halt_poll_ns;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 72c4e6b39389..962b91ac2064 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -3686,6 +3686,7 @@ static long kvm_vcpu_ioctl(struct file *filp,
>  	int r;
>  	struct kvm_fpu *fpu = NULL;
>  	struct kvm_sregs *kvm_sregs = NULL;
> +	struct kvm *kvm = vcpu->kvm;

If you're going to bother grabbing kvm, replace the instances below that also do
vcpu->kvm.

>  
>  	if (vcpu->kvm->mm != current->mm || vcpu->kvm->vm_dead)
>  		return -EIO;
> @@ -3723,6 +3724,14 @@ static long kvm_vcpu_ioctl(struct file *filp,
>  			if (oldpid)
>  				synchronize_rcu();
>  			put_pid(oldpid);
> +
> +			/*
> +			 * Since we land here even on the first vCPU run,
> +			 * we can mark that the VM has started running.

Please avoid "we", "us", etc..

"vm_started" is also ambiguous.  If we end up with a flag, then I would prefer a
much more literal name, a la created_vcpus, e.g. ran_vcpus or something.

> +			 */
> +			mutex_lock(&kvm->lock);

This adds unnecessary lock contention when running vCPUs.  The naive solution
would be:
			if (!kvm->vm_started) {
				...
			}

> +			kvm->vm_started = true;
> +			mutex_unlock(&kvm->lock);

Lastly, why is this in generic KVM?

>  		}
>  		r = kvm_arch_vcpu_ioctl_run(vcpu);
>  		trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
> -- 
> 2.34.1.448.ga2b2bfdf31-goog
> 
> _______________________________________________
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> [email-masked]
> https://lists.cs.columbia.edu/mailman/listinfo/kvmarm

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