netdev - Re: [PATCH net-next 12/14] vsock/vmci: register vmci_transport only when VMCI guest/host are active

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Date:   Tue, 12 Nov 2019 11:42:01 +0100
From:   Stefano Garzarella <sgarzare@...hat.com>
To:     Jorgen Hansen <jhansen@...are.com>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jason Wang <jasowang@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Dexuan Cui <decui@...rosoft.com>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Sasha Levin <sashal@...nel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Arnd Bergmann <arnd@...db.de>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        "linux-hyperv@...r.kernel.org" <linux-hyperv@...r.kernel.org>,
        "K. Y. Srinivasan" <kys@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        "virtualization@...ts.linux-foundation.org" 
        <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH net-next 12/14] vsock/vmci: register vmci_transport only
 when VMCI guest/host are active

On Tue, Nov 12, 2019 at 10:03:54AM +0000, Jorgen Hansen wrote:
> > From: Stefano Garzarella [mailto:sgarzare@...hat.com]
> > Sent: Monday, November 11, 2019 6:31 PM
> > On Mon, Nov 11, 2019 at 04:27:28PM +0000, Jorgen Hansen wrote:
> > > > From: Stefano Garzarella [mailto:sgarzare@...hat.com]
> > > > Sent: Wednesday, October 23, 2019 11:56 AM
> > > >
> > > > To allow other transports to be loaded with vmci_transport,
> > > > we register the vmci_transport as G2H or H2G only when a VMCI guest
> > > > or host is active.
> > > >
> > > > To do that, this patch adds a callback registered in the vmci driver
> > > > that will be called when a new host or guest become active.
> > > > This callback will register the vmci_transport in the VSOCK core.
> > > > If the transport is already registered, we ignore the error coming
> > > > from vsock_core_register().
> > >
> > > So today this is mainly an issue for the VMCI vsock transport, because
> > > VMCI autoloads with vsock (and with this solution it can continue to
> > > do that, so none of our old products break due to changed behavior,
> > > which is great).
> > 
> > I tried to not break anything :-)
> > 
> > >                  Shouldn't vhost behave similar, so that any module
> > > that registers a h2g transport only does so if it is in active use?
> > >
> > 
> > The vhost-vsock module will load when the first hypervisor open
> > /dev/vhost-vsock, so in theory, when there's at least one active user.
> 
> Ok, sounds good then. 
> 
> > 
> > >
> > > > --- a/drivers/misc/vmw_vmci/vmci_host.c
> > > > +++ b/drivers/misc/vmw_vmci/vmci_host.c
> > > > @@ -108,6 +108,11 @@ bool vmci_host_code_active(void)
> > > >  	     atomic_read(&vmci_host_active_users) > 0);
> > > >  }
> > > >
> > > > +int vmci_host_users(void)
> > > > +{
> > > > +	return atomic_read(&vmci_host_active_users);
> > > > +}
> > > > +
> > > >  /*
> > > >   * Called on open of /dev/vmci.
> > > >   */
> > > > @@ -338,6 +343,8 @@ static int vmci_host_do_init_context(struct
> > > > vmci_host_dev *vmci_host_dev,
> > > >  	vmci_host_dev->ct_type = VMCIOBJ_CONTEXT;
> > > >  	atomic_inc(&vmci_host_active_users);
> > > >
> > > > +	vmci_call_vsock_callback(true);
> > > > +
> > >
> > > Since we don't unregister the transport if user count drops back to 0, we
> > could
> > > just call this the first time, a VM is powered on after the module is loaded.
> > 
> > Yes, make sense. can I use the 'vmci_host_active_users' or is better to
> > add a new 'vmci_host_vsock_loaded'?
> > 
> > My doubt is that vmci_host_active_users can return to 0, so when it returns
> > to 1, we call vmci_call_vsock_callback() again.
> 
> vmci_host_active_users can drop to 0 and then increase again, so having a flag
> indicating whether the callback has been invoked would ensure that it is only
> called once.

I agree, I will use a dedicated flag, maybe in the
vmci_call_vsock_callback(), since it can be called or during the
vmci_host_do_init_context() or when the callback is registered.

Thanks,
Stefano