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From:   Jorgen Hansen <jhansen@vmware.com>
To:     'Stefano Garzarella' <sgarzare@redhat.com>
CC:     "netdev@vger.kernel.org" <netdev@vger.kernel.org>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Jason Wang <jasowang@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        Dexuan Cui <decui@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
        Sasha Levin <sashal@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
        "K. Y. Srinivasan" <kys@microsoft.com>,
        Stephen Hemminger <sthemmin@microsoft.com>,
        "virtualization@lists.linux-foundation.org" 
        <virtualization@lists.linux-foundation.org>
Subject: RE: [PATCH net-next 12/14] vsock/vmci: register vmci_transport only
 when VMCI guest/host are active
Thread-Topic: [PATCH net-next 12/14] vsock/vmci: register vmci_transport only
 when VMCI guest/host are active
Thread-Index: AQHViYhpRc2q1qJxVEORBsxH5VDhoaeGP/hggAAYgICAARRFEA==
Date:   Tue, 12 Nov 2019 10:03:54 +0000
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References: <20191023095554.11340-1-sgarzare@redhat.com>
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 <MWHPR05MB3376266BC6AE9E6E0B75F1A1DA740@MWHPR05MB3376.namprd05.prod.outlook.com>
 <20191111173053.erwfzawioxje635o@steredhat>
In-Reply-To: <20191111173053.erwfzawioxje635o@steredhat>
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> From: Stefano Garzarella [mailto:sgarzare@redhat.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@redhat.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).
>=20
> I tried to not break anything :-)
>=20
> >                  Shouldn't vhost behave similar, so that any module
> > that registers a h2g transport only does so if it is in active use?
> >
>=20
> 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.=20

>=20
> >
> > > --- 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 =3D 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 l=
oaded.
>=20
> Yes, make sense. can I use the 'vmci_host_active_users' or is better to
> add a new 'vmci_host_vsock_loaded'?
>=20
> My doubt is that vmci_host_active_users can return to 0, so when it retur=
ns
> to 1, we call vmci_call_vsock_callback() again.

vmci_host_active_users can drop to 0 and then increase again, so having a f=
lag
indicating whether the callback has been invoked would ensure that it is on=
ly
called once.

Thanks,
Jorgen