Re: [Xen-devel] [PATCH] tools: specifically enable VirtFS in Linux QEMU builds

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: George Dunlap [mailto:george.dunlap@xxxxxxxxxx]
> Sent: 06 September 2018 13:52
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>; George Dunlap
> <George.Dunlap@xxxxxxxxxx>; Ian Jackson <Ian.Jackson@xxxxxxxxxx>; Jan
> Beulich <jbeulich@xxxxxxxx>; Julien Grall <julien.grall@xxxxxxx>; Konrad
> Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>; Stefano Stabellini
> <sstabellini@xxxxxxxxxx>; Tim (Xen.org) <tim@xxxxxxx>; Wei Liu
> <wei.liu2@xxxxxxxxxx>
> Subject: Re: [PATCH] tools: specifically enable VirtFS in Linux QEMU builds
> 
> On 09/06/2018 01:06 PM, Paul Durrant wrote:
> > 9pfs support has been a documented feature since Xen 4.9, but QEMU will
> > not be built with backend support unless libcap and libattr dev packages
> > are installed.
> >
> > This patch modifies the README to call out those packages as pre-
> requisites
> > for Linux builds and specifically enables VirtFS in the configure line
> > for QEMU so that an error message is displayed if they are missing.
> >
> > Signed-off-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
> > ---
> > Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> > Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> > Cc: Ian Jackson <ian.jackson@xxxxxxxxxxxxx>
> > Cc: Jan Beulich <jbeulich@xxxxxxxx>
> > Cc: Julien Grall <julien.grall@xxxxxxx>
> > Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > Cc: Tim Deegan <tim@xxxxxxx>
> > Cc: Wei Liu <wei.liu2@xxxxxxxxxx>
> > ---
> >  README         |  2 ++
> >  tools/Makefile | 11 +++++++++++
> >  2 files changed, 13 insertions(+)
> >
> > diff --git a/README b/README
> > index 4b95b21c7b..1a4e4b2c1b 100644
> > --- a/README
> > +++ b/README
> > @@ -56,6 +56,8 @@ provided by your OS distributor:
> >        greater.
> >      * Development install of GLib v2.0 (e.g. libglib2.0-dev)
> >      * Development install of Pixman (e.g. libpixman-1-dev)
> > +    * Development install of libcap (e.g. libcap-dev) [Linux only]
> > +    * Development install of libattr (e.g. libattr1-dev) [Linux only]
> >      * pkg-config
> >      * bridge-utils package (/sbin/brctl)
> >      * iproute package (/sbin/ip)
> > diff --git a/tools/Makefile b/tools/Makefile
> > index 67977ad850..e74efb8a6e 100644
> > --- a/tools/Makefile
> > +++ b/tools/Makefile
> > @@ -216,6 +216,11 @@ else
> >  QEMU_XEN_ENABLE_DEBUG :=
> >  endif
> >
> > +#
> > +# 9pfs support is a documented feature but it depends on a QEMU with
> > +# VirtFS enabled. However VirtFS is a Linux-only option so only enable
> > +# it for Linux builds.
> > +#
> >  subdir-all-qemu-xen-dir: qemu-xen-dir-find
> >     unset MAKELEVEL; \
> >     if test -d $(QEMU_UPSTREAM_LOC) ; then \
> > @@ -232,10 +237,16 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
> >     else \
> >             enable_trace_backend='' ; \
> >     fi ; \
> > +   if [ "$(CONFIG_Linux)" = "y" ]; then \
> > +           enable_virtfs='--enable-virtfs' ; \
> > +   else \
> > +           enable_virtfs='' ; \
> > +   fi ; \
> 
> 9pfs is still a reasonably "niche" feature; I don't think we want to
> force people to enable it if they don't expect to use it.
> 
> Is it the case that if we have libcap and libattr, that qemu will enable
> 9pfs automatically?  If so we should just document that.

Yes, that is the case but it does seem weird that we have a documented feature 
which is not being built in by default, because we don't call out or check for 
the necessary pre-requisites.

> 
> Another option would be to have "--enable-9pfs" option in the tools
> configure, which would then pass this on to qemu (and thus fail if the
> requisite librares aren't present), but I'm not sure if having a load of
> "--enable-*" at the top level is really what we want for this.
> 

That is something I wondered about. Really I just don't want people to be 
scrobbling around wondering why, having built and installed Xen and QEMU put a 
'9p' line in their config and then kicked off their guest, it simply doesn't 
work.

  Paul

>  -George
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