Re: [PATCH v4 10/11] 9p: darwin: meson: Allow VirtFS on Darwin

[Greg Kurz <groug@kaod.org>]

On Mon, 07 Feb 2022 15:15:46 +0100
Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Montag, 7. Februar 2022 02:05:32 CET Will Cohen wrote:
> > On Sun, Feb 6, 2022 at 4:22 PM Philippe Mathieu-Daudé <f4bug@amsat.org>
> > 
> > wrote:
> > > On 6/2/22 21:07, Will Cohen wrote:
> > > > From: Keno Fischer <keno@juliacomputing.com>
> > > > 
> > > > Signed-off-by: Keno Fischer <keno@juliacomputing.com>
> > > > [Michael Roitzsch: - Rebase for NixOS]
> > > > Signed-off-by: Michael Roitzsch <reactorcontrol@icloud.com>
> > > > [Will Cohen: - Rebase to master]
> > > > Signed-off-by: Will Cohen <wwcohen@gmail.com>
> > > > Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
> > > > [Will Cohen: - Add check for pthread_fchdir_np to virtfs]
> > > > Signed-off-by: Will Cohen <wwcohen@gmail.com>
> > > > ---
> > > > 
> > > >   fsdev/meson.build |  1 +
> > > >   meson.build       | 14 ++++++++++----
> > > >   2 files changed, 11 insertions(+), 4 deletions(-)
> > > > 
> > > > -have_virtfs_proxy_helper = have_virtfs and have_tools
> > > > +have_virtfs_proxy_helper = targetos == 'linux' and have_virtfs and
> > > 
> > > have_tools
> > > 
> > > Why do you restrict the proxy-helper to Linux?
> >
> > Only because porting the proxy-helper to macOS is outside the scope of this
> > particular patch. While some initial concepts around it have been
> > considered by some of the contributors to this patch, those implementations
> > weren't tested enough and the security implications weren't considered in
> > full. We assume that this could be an additional implementation later on,
> > if the functionality is considered important down the road.
> 
> In general that's fine with me. I would have probably made that
> "targetos != 'darwin'" instead of "targetos == 'linux'", but I leave that up 
> to you.
> 
> On the long term we will probably deprecate the 9p 'proxy' fs driver anyway. 
> While it had some good ideas, being realistic though: nobody has worked on the 
> 9p proxy driver/backend for many years and it is not in good shape.
> 

It definitely isn't indeed. Also it is super slow by design
since the round trip of a 9p request involves QEMU on both entry
and exit:

   [guest] --> [QEMU]--> [virtfs-proxy-helper]-->[QEMU]-->[guest]

A more modern and efficient approach would be to have a vhost-user-9p
implementation : requests would be directly handled by the external
process, without QEMU hops. But this a fair amount of work.

> I can imagine that due to the ground being laid by these series, that we will 
> also open 9p for BSD, but that should be done a bit later and hence does not 
> belong into these series.
> 
> But once again: it would not have hurt to make your intentions clear either in 
> the commit log or by in-source comment. :)
> 
> Best regards,
> Christian Schoenebeck
> 
>