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

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Message-ID: <CAAeT=FwfjUiaWAJHCEqTCyu9PRr+JEKx1v0P-CJ=2-yGBvaGbA@mail.gmail.com>
Date:   Mon, 10 Jan 2022 16:03:41 -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

On Fri, Jan 7, 2022 at 3:43 PM Raghavendra Rao Ananta
<rananta@...gle.com> wrote:
>
> Hi Reiji,
>
> On Thu, Jan 6, 2022 at 10:07 PM Reiji Watanabe <reijiw@...gle.com> wrote:
> >
> > 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 (?).
> >
> I always struggle with the names; but if you feel that 'ran_once'
> makes more sense for a reader, I can change it.

I would prefer 'ran_once'.


> > >  };
> > >
> > >  #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.
> >
> I've been going back and forth on this one. Initially I considered
> simply going with atomic_t, but the patch 4/11 (KVM: arm64: Setup a
> framework for hypercall bitmap firmware registers)
> kvm_arm_set_fw_reg_bmap()'s implementation felt like we need a lock to
> have the whole 'is the register busy?' operation atomic. But, that's
> just one of the applications.

I understand why you need the code to get the lock here with the
current implementation.
But, since the code just set the one field (vm_started) with the lock,
I thought the intention of getting the lock might not be so obvious.
(But, maybe clear enough looking at the code in the patch-4)

Thanks,
Reiji


> > 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>
> >
> Glad that it's helping you as well and thanks for the review.
>
> Regards,
> Raghavendra
>
> > 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
> > >