Linux Virtualization - Nov 2019 - [PATCH net-next 4/6] vsock: add vsock_loopback transport

On Thu, Nov 21, 2019 at 10:59:48AM +0100, Stefano Garzarella
wrote:
> On Thu, Nov 21, 2019 at 09:34:58AM +0000, Stefan Hajnoczi wrote:
> > On Tue, Nov 19, 2019 at 12:01:19PM +0100, Stefano Garzarella wrote:
> > 
> > Ideas for long-term changes below.
> > 
> > Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
> > 
> 
> Thanks for reviewing!
> 
> > > diff --git a/MAINTAINERS b/MAINTAINERS
> > > index 760049454a23..c2a3dc3113ba 100644
> > > --- a/MAINTAINERS
> > > +++ b/MAINTAINERS
> > > @@ -17239,6 +17239,7 @@ F:	net/vmw_vsock/diag.c
> > >  F:	net/vmw_vsock/af_vsock_tap.c
> > >  F:	net/vmw_vsock/virtio_transport_common.c
> > >  F:	net/vmw_vsock/virtio_transport.c
> > > +F:	net/vmw_vsock/vsock_loopback.c
> > >  F:	drivers/net/vsockmon.c
> > >  F:	drivers/vhost/vsock.c
> > >  F:	tools/testing/vsock/
> > 
> > At this point you are most active in virtio-vsock and I am reviewing
> > patches on a best-effort basis.  Feel free to add yourself as
> > maintainer.
> > 
> 
> Sure, I'd be happy to maintain it.
> 
> > > diff --git a/net/vmw_vsock/vsock_loopback.c
b/net/vmw_vsock/vsock_loopback.c
> > > new file mode 100644
> > > index 000000000000..3d1c1a88305f
> > > --- /dev/null
> > > +++ b/net/vmw_vsock/vsock_loopback.c
> > > @@ -0,0 +1,217 @@
> > > +// SPDX-License-Identifier: GPL-2.0-only
> > > +/*
> > > + * loopback transport for vsock using virtio_transport_common
APIs
> > > + *
> > > + * Copyright (C) 2013-2019 Red Hat, Inc.
> > > + * Author: Asias He <asias at redhat.com>
> > > + *         Stefan Hajnoczi <stefanha at redhat.com>
> > > + *         Stefano Garzarella <sgarzare at redhat.com>
> > > + *
> > > + */
> > > +#include <linux/spinlock.h>
> > > +#include <linux/module.h>
> > > +#include <linux/list.h>
> > > +#include <linux/virtio_vsock.h>
> > 
> > Is it time to rename the generic functionality in
> > virtio_transport_common.c?  This doesn't have anything to do with
virtio
> > :).
> > 
> 
> Completely agree, new transports could use it to handle the protocol
without
> reimplementing things already done.
> 
> > > +
> > > +static struct workqueue_struct *vsock_loopback_workqueue;
> > > +static struct vsock_loopback *the_vsock_loopback;
> > 
> > the_vsock_loopback could be a static global variable (not a pointer)
and
> > vsock_loopback_workqueue could also be included in the struct.
> > 
> > The RCU pointer is really a way to synchronize
vsock_loopback_send_pkt()
> > and vsock_loopback_cancel_pkt() with module exit.  There is no other
> > reason for using a pointer.
> > 
> > It's cleaner to implement the synchronization once in af_vsock.c
(or
> > virtio_transport_common.c) instead of making each transport do it.
> > Maybe try_module_get() and related APIs provide the necessary
semantics
> > so that core vsock code can hold the transport module while it's
being
> > used to send/cancel a packet.
> 
> Right, the module cannot be unloaded until open sockets, so here the
> synchronization is not needed.
> 
> The synchronization come from virtio-vsock device that can be
> hot-unplugged while sockets are still open, but that can't happen here.
> 
> I will remove the pointers and RCU in the v2.
> 
> Can I keep your R-b or do you prefer to watch v2 first?
> 
> > 
> > > +MODULE_ALIAS_NETPROTO(PF_VSOCK);
> > 
> > Why does this module define the alias for PF_VSOCK?  Doesn't
another
> > module already define this alias?
> 
> It is a way to load this module when PF_VSOCK is starting to be used.
> MODULE_ALIAS_NETPROTO(PF_VSOCK) is already defined in vmci_transport
> and hyperv_transport. IIUC it is used for the same reason.
> 
> In virtio_transport we don't need it because it will be loaded when
> the PCI device is discovered.
> 
> Do you think there's a better way?
> Should I include the vsock_loopback transport directly in af_vsock
> without creating a new module?
> 
That last thing I said may not be possible:
I remembered that I tried, but DEPMOD found a cyclic dependency because
vsock_transport use virtio_transport_common that use vsock, so if I
include vsock_transport in the vsock module, DEPMOD is not happy.

Do you think it's okay in this case to keep MODULE_ALIAS_NETPROTO(PF_VSOCK)
or is there a better way?

Thanks,
Stefano

On Thu, Nov 21, 2019 at 04:25:17PM +0100, Stefano Garzarella
wrote:
> On Thu, Nov 21, 2019 at 10:59:48AM +0100, Stefano Garzarella wrote:
> > On Thu, Nov 21, 2019 at 09:34:58AM +0000, Stefan Hajnoczi wrote:
> > > On Tue, Nov 19, 2019 at 12:01:19PM +0100, Stefano Garzarella
wrote:
> > > > +static struct workqueue_struct *vsock_loopback_workqueue;
> > > > +static struct vsock_loopback *the_vsock_loopback;
> > > 
> > > the_vsock_loopback could be a static global variable (not a
pointer) and
> > > vsock_loopback_workqueue could also be included in the struct.
> > > 
> > > The RCU pointer is really a way to synchronize
vsock_loopback_send_pkt()
> > > and vsock_loopback_cancel_pkt() with module exit.  There is no
other
> > > reason for using a pointer.
> > > 
> > > It's cleaner to implement the synchronization once in
af_vsock.c (or
> > > virtio_transport_common.c) instead of making each transport do
it.
> > > Maybe try_module_get() and related APIs provide the necessary
semantics
> > > so that core vsock code can hold the transport module while
it's being
> > > used to send/cancel a packet.
> > 
> > Right, the module cannot be unloaded until open sockets, so here the
> > synchronization is not needed.
> > 
> > The synchronization come from virtio-vsock device that can be
> > hot-unplugged while sockets are still open, but that can't happen
here.
> > 
> > I will remove the pointers and RCU in the v2.
> > 
> > Can I keep your R-b or do you prefer to watch v2 first?
I'd like to review v2.
> > > > +MODULE_ALIAS_NETPROTO(PF_VSOCK);
> > > 
> > > Why does this module define the alias for PF_VSOCK?  Doesn't
another
> > > module already define this alias?
> > 
> > It is a way to load this module when PF_VSOCK is starting to be used.
> > MODULE_ALIAS_NETPROTO(PF_VSOCK) is already defined in vmci_transport
> > and hyperv_transport. IIUC it is used for the same reason.
> > 
> > In virtio_transport we don't need it because it will be loaded
when
> > the PCI device is discovered.
> > 
> > Do you think there's a better way?
> > Should I include the vsock_loopback transport directly in af_vsock
> > without creating a new module?
> > 
> 
> That last thing I said may not be possible:
> I remembered that I tried, but DEPMOD found a cyclic dependency because
> vsock_transport use virtio_transport_common that use vsock, so if I
> include vsock_transport in the vsock module, DEPMOD is not happy.
> 
> Do you think it's okay in this case to keep
MODULE_ALIAS_NETPROTO(PF_VSOCK)
> or is there a better way?
The reason I asked is because the semantics of duplicate module aliases
aren't clear to me.  Do all modules with the same alias get loaded?
Or just the first?  Or ...?

Stefan
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On Fri, Nov 22, 2019 at 09:25:46AM +0000, Stefan Hajnoczi
wrote:
> On Thu, Nov 21, 2019 at 04:25:17PM +0100, Stefano Garzarella wrote:
> > On Thu, Nov 21, 2019 at 10:59:48AM +0100, Stefano Garzarella wrote:
> > > On Thu, Nov 21, 2019 at 09:34:58AM +0000, Stefan Hajnoczi wrote:
> > > > On Tue, Nov 19, 2019 at 12:01:19PM +0100, Stefano Garzarella
wrote:
> > > > > +static struct workqueue_struct
*vsock_loopback_workqueue;
> > > > > +static struct vsock_loopback *the_vsock_loopback;
> > > > 
> > > > the_vsock_loopback could be a static global variable (not a
pointer) and
> > > > vsock_loopback_workqueue could also be included in the
struct.
> > > > 
> > > > The RCU pointer is really a way to synchronize
vsock_loopback_send_pkt()
> > > > and vsock_loopback_cancel_pkt() with module exit.  There is
no other
> > > > reason for using a pointer.
> > > > 
> > > > It's cleaner to implement the synchronization once in
af_vsock.c (or
> > > > virtio_transport_common.c) instead of making each transport
do it.
> > > > Maybe try_module_get() and related APIs provide the
necessary semantics
> > > > so that core vsock code can hold the transport module while
it's being
> > > > used to send/cancel a packet.
> > > 
> > > Right, the module cannot be unloaded until open sockets, so here
the
> > > synchronization is not needed.
> > > 
> > > The synchronization come from virtio-vsock device that can be
> > > hot-unplugged while sockets are still open, but that can't
happen here.
> > > 
> > > I will remove the pointers and RCU in the v2.
> > > 
> > > Can I keep your R-b or do you prefer to watch v2 first?
> 
> I'd like to review v2.
> 
Sure!
> > > > > +MODULE_ALIAS_NETPROTO(PF_VSOCK);
> > > > 
> > > > Why does this module define the alias for PF_VSOCK? 
Doesn't another
> > > > module already define this alias?
> > > 
> > > It is a way to load this module when PF_VSOCK is starting to be
used.
> > > MODULE_ALIAS_NETPROTO(PF_VSOCK) is already defined in
vmci_transport
> > > and hyperv_transport. IIUC it is used for the same reason.
> > > 
> > > In virtio_transport we don't need it because it will be
loaded when
> > > the PCI device is discovered.
> > > 
> > > Do you think there's a better way?
> > > Should I include the vsock_loopback transport directly in
af_vsock
> > > without creating a new module?
> > > 
> > 
> > That last thing I said may not be possible:
> > I remembered that I tried, but DEPMOD found a cyclic dependency
because
> > vsock_transport use virtio_transport_common that use vsock, so if I
> > include vsock_transport in the vsock module, DEPMOD is not happy.
> > 
> > Do you think it's okay in this case to keep
MODULE_ALIAS_NETPROTO(PF_VSOCK)
> > or is there a better way?
> 
> The reason I asked is because the semantics of duplicate module aliases
> aren't clear to me.  Do all modules with the same alias get loaded?
> Or just the first?  Or ...?
It wasn't clear to me either, but when I tried, I saw that all modules
with the same alias got loaded.

Stefano