Re: [Qemu-devel] [PATCH 3/8] xen/9pfs: introduce Xen 9pfs backend

[Greg Kurz <groug@kaod.org>]

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On Thu, 9 Mar 2017 18:54:26 +0100
Greg Kurz <groug@kaod.org> wrote:

> On Mon,  6 Mar 2017 18:12:43 -0800
> Stefano Stabellini <sstabellini@kernel.org> wrote:
> 
> > Introduce the Xen 9pfs backend: add struct XenDevOps to register as a
> > Xen backend and add struct V9fsTransport to register as v9fs transport.
> > 
> > All functions are empty stubs for now.
> > 
> > Signed-off-by: Stefano Stabellini <stefano@aporeto.com>
> > CC: anthony.perard@citrix.com
> > CC: jgross@suse.com
> > CC: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
> > CC: Greg Kurz <groug@kaod.org>
> > ---
> >  hw/9pfs/xen-9p-backend.c | 96 ++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 96 insertions(+)
> >  create mode 100644 hw/9pfs/xen-9p-backend.c
> > 
> > diff --git a/hw/9pfs/xen-9p-backend.c b/hw/9pfs/xen-9p-backend.c
> > new file mode 100644
> > index 0000000..924fb64
> > --- /dev/null
> > +++ b/hw/9pfs/xen-9p-backend.c
> > @@ -0,0 +1,96 @@
> > +/*
> > + * Xen 9p backend
> > + *
> > + * Copyright Aporeto 2017
> > + *
> > + * Authors:
> > + *  Stefano Stabellini <stefano@aporeto.com>
> > + *
> > + */
> > +
> > +#include "qemu/osdep.h"
> > +
> > +#include "hw/hw.h"
> > +#include "hw/9pfs/9p.h"
> > +#include "hw/xen/xen_backend.h"
> > +#include "xen_9pfs.h"
> > +#include "qemu/config-file.h"
> > +#include "fsdev/qemu-fsdev.h"
> > +
> > +struct Xen9pfsDev {
> > +    struct XenDevice xendev;  /* must be first */
> > +};  
> 
> According to HACKING, this should be:
> 
> typedef struct Xen9pfsDev {
>     struct XenDevice xendev;  /* must be first */
> } Xen9pfsDev;
> 
> and...
> 
> > +
> > +static ssize_t xen_9pfs_pdu_vmarshal(V9fsPDU *pdu,
> > +                                     size_t offset,
> > +                                     const char *fmt,
> > +                                     va_list ap)
> > +{
> > +    return 0;
> > +}
> > +
> > +static ssize_t xen_9pfs_pdu_vunmarshal(V9fsPDU *pdu,
> > +                                       size_t offset,
> > +                                       const char *fmt,
> > +                                       va_list ap)
> > +{
> > +    return 0;
> > +}
> > +
> > +static void xen_9pfs_init_out_iov_from_pdu(V9fsPDU *pdu,
> > +                                           struct iovec **piov,
> > +                                           unsigned int *pniov)
> > +{
> > +}
> > +
> > +static void xen_9pfs_init_in_iov_from_pdu(V9fsPDU *pdu,
> > +                                          struct iovec **piov,
> > +                                          unsigned int *pniov,
> > +                                          size_t size)
> > +{
> > +}
> > +
> > +static void xen_9pfs_push_and_notify(V9fsPDU *pdu)
> > +{
> > +}
> > +
> > +static const struct V9fsTransport xen_9p_transport = {
> > +    .pdu_vmarshal = xen_9pfs_pdu_vmarshal,
> > +    .pdu_vunmarshal = xen_9pfs_pdu_vunmarshal,
> > +    .init_in_iov_from_pdu = xen_9pfs_init_in_iov_from_pdu,
> > +    .init_out_iov_from_pdu = xen_9pfs_init_out_iov_from_pdu,
> > +    .push_and_notify = xen_9pfs_push_and_notify,
> > +};
> > +
> > +static int xen_9pfs_init(struct XenDevice *xendev)
> > +{
> > +    return 0;
> > +}
> > +
> > +static int xen_9pfs_free(struct XenDevice *xendev)
> > +{
> > +    return -1;
> > +}
> > +
> > +static int xen_9pfs_connect(struct XenDevice *xendev)
> > +{
> > +    return 0;
> > +}
> > +
> > +static void xen_9pfs_alloc(struct XenDevice *xendev)
> > +{
> > +}
> > +
> > +static void xen_9pfs_disconnect(struct XenDevice *xendev)
> > +{
> > +}
> > +
> > +struct XenDevOps xen_9pfs_ops = {
> > +    .size       = sizeof(struct Xen9pfsDev),  
> 
> ... s/struct //
> 
> > +    .flags      = DEVOPS_FLAG_NEED_GNTDEV,
> > +    .alloc      = xen_9pfs_alloc,
> > +    .init       = xen_9pfs_init,
> > +    .initialise    = xen_9pfs_connect,
> > +    .disconnect = xen_9pfs_disconnect,
> > +    .free       = xen_9pfs_free,
> > +};  
> 
> With the above comments addressed:
> 
> Reviewed-by: Greg Kurz <groug@kaod.org>

Hmm... there's still a problem actually. This patch breaks build for some
targets with '--enable-xen --enable-virtfs':

  LINK    cris-softmmu/qemu-system-cris
../hw/xen/xen_backend.o: In function `xen_be_register_common':
/home/greg/Work/qemu/qemu-9p/hw/xen/xen_backend.c:588: undefined reference to `xen_9pfs_ops'

This happens because only targets that support virtio and PCI pull the
9pfs/fsdev object files. This is an effort to have smaller QEMU binaries,
based on the historical dependency of 9pfs on virtio. This patch breaks
this assumption if CONFIG_XEN_BACKEND is defined but the target doesn't
define CONFIG_VIRTIO and CONFIG_PCI.

Since 9pfs can now be used with another transport, I guess that the
dependency on virtio and PCI isn't right anymore. The new condition
for targets to support 9pfs should rather be something like:

CONFIG_VIRTFS == y && CONFIG_SOFTMMU == y && (CONFIG_VIRTIO == y || CONFIG_XEN_BACKEND == y)

This would cause all targets to pull the 9pfs/fsdev object files though.

So I have a question: do all targets need to support the Xen backend, really ?

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