linux-kernel - Re: [RFC PATCH v3 01/11] KVM: Capture VM start

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Date:   Thu, 6 Jan 2022 22:06:53 -0800
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Raghavendra Rao Ananta <rananta@...gle.com>
Cc:     Marc Zyngier <maz@...nel.org>, Andrew Jones <drjones@...hat.com>,
        James Morse <james.morse@....com>,
        Alexandru Elisei <alexandru.elisei@....com>,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Peter Shier <pshier@...gle.com>,
        Ricardo Koller <ricarkol@...gle.com>,
        Oliver Upton <oupton@...gle.com>,
        Jing Zhang <jingzhangos@...gle.com>,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        kvmarm@...ts.cs.columbia.edu, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start

Hi Raghu,

On Tue, Jan 4, 2022 at 11:49 AM Raghavendra Rao Ananta
<rananta@...gle.com> wrote:
>
> Capture the start of the KVM VM, which is basically the
> 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.
>
> Signed-off-by: Raghavendra Rao Ananta <rananta@...gle.com>
> ---
>  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;

Since KVM_RUN on any vCPUs doesn't necessarily mean that the VM
started yet, the name might be a bit misleading IMHO.  I would
think 'has_run_once' or 'ran_once' might be more clear (?).


>  };
>
>  #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)
> +
>  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 (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.
> +                        */

It might be nicer to add a comment why the code below gets kvm->lock.

Anyway, the patch generally looks good to me, and thank you
for making this change (it works for my purpose as well).

Reviewed-by: Reiji Watanabe <reijiw@...gle.com>

Thanks,
Reiji


> +                       mutex_lock(&kvm->lock);
> +                       kvm->vm_started = true;
> +                       mutex_unlock(&kvm->lock);
>                 }
>                 r = kvm_arch_vcpu_ioctl_run(vcpu);
>                 trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
> --
> 2.34.1.448.ga2b2bfdf31-goog
>