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In-Reply-To: <Ydjje8qBOP3zDOZi@google.com>
From:   Raghavendra Rao Ananta <rananta@google.com>
Date:   Mon, 10 Jan 2022 15:23:46 -0800
Message-ID: <CAJHc60ziKv6P4ZmpLXrv+s4DrrDtOwuQRAc4bKcrbR3aNAK5mQ@mail.gmail.com>
Subject: Re: [RFC PATCH v3 01/11] KVM: Capture VM start
To:     Sean Christopherson <seanjc@google.com>
Cc:     Marc Zyngier <maz@kernel.org>, Andrew Jones <drjones@redhat.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        kvm@vger.kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
        Peter Shier <pshier@google.com>, linux-kernel@vger.kernel.org,
        Paolo Bonzini <pbonzini@redhat.com>,
        Will Deacon <will@kernel.org>, kvmarm@lists.cs.columbia.edu,
        linux-arm-kernel@lists.infradead.org
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Precedence: bulk

On Fri, Jan 7, 2022 at 5:06 PM Sean Christopherson <seanjc@google.com> wrote:
>
> 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 <rananta@google.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;
> >  };
> >
> >  #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.
>
No specific reason for creating a macro as such. I can remove it if it
feels noisy.
> > +
> >  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) {
>                                 ...
>                         }
>
Not sure if I understood the solution..

> > +                     kvm->vm_started = true;
> > +                     mutex_unlock(&kvm->lock);
>
> Lastly, why is this in generic KVM?
>
The v1 of the series originally had it in the arm specific code.
However, I was suggested to move it to the generic code since the book
keeping is not arch specific and could be helpful to others too [1].

Thanks for the review. I'll add your other comments as well.

Regards,
Raghavendra

[1]: https://lore.kernel.org/kvmarm/YYMKphExkqttn2w0@google.com/

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