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

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Date:   Mon, 11 Nov 2019 18:30:53 +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 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.

> 
> > --- 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.

Thanks,
Stefano