Re: [Qemu-devel] [RFC v2 11/32] vhub: Open userfaultfd

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> On Thu, Aug 24, 2017 at 08:27:09PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > 
> > Open a userfaultfd (on a postcopy_advise) and send it back in
> > the reply to the qemu for it to monitor.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > ---
> >  contrib/libvhost-user/libvhost-user.c | 26 +++++++++++++++++++++++---
> >  contrib/libvhost-user/libvhost-user.h |  3 +++
> >  2 files changed, 26 insertions(+), 3 deletions(-)
> > 
> > diff --git a/contrib/libvhost-user/libvhost-user.c b/contrib/libvhost-user/libvhost-user.c
> > index 47884c0a15..f9b5b12b28 100644
> > --- a/contrib/libvhost-user/libvhost-user.c
> > +++ b/contrib/libvhost-user/libvhost-user.c
> > @@ -15,6 +15,7 @@
> >  
> >  #include <qemu/osdep.h>
> >  #include <sys/eventfd.h>
> > +#include <sys/syscall.h>
> >  #include <linux/vhost.h>
> >  
> >  #include "qemu/atomic.h"
> > @@ -773,11 +774,30 @@ vu_set_vring_enable_exec(VuDev *dev, VhostUserMsg *vmsg)
> >  static bool
> >  vu_set_postcopy_advise(VuDev *dev, VhostUserMsg *vmsg)
> >  {
> > -    /* TODO: Open ufd, pass it back in the request
> > -     * TODO: Add addresses 
> > -     */
> > +    struct uffdio_api api_struct;
> > +
> > +    dev->postcopy_ufd = syscall(__NR_userfaultfd, O_CLOEXEC | O_NONBLOCK);
> > +    /* TODO: Add addresses */
> >      vmsg->payload.u64 = 0xcafe;
> >      vmsg->size = sizeof(vmsg->payload.u64);
> > +
> > +    if (dev->postcopy_ufd == -1) {
> > +        vu_panic(dev, "Userfaultfd not available: %s", strerror(errno));
> > +        goto out;
> 
> We got error but still goto out?  I feel like we should reply with
> some kind of error code when any error happens.

See that the out: code returns the dev->postcopy_ufd to qemu
and in this case it's -1 so it is already flagged as an error.

> > +    }
> > +    api_struct.api = UFFD_API;
> > +    api_struct.features = 0;
> > +    if (ioctl(dev->postcopy_ufd, UFFDIO_API, &api_struct)) {
> > +        vu_panic(dev, "Failed UFFDIO_API: %s", strerror(errno));
> > +        close(dev->postcopy_ufd);
> > +        dev->postcopy_ufd = -1;
> > +        goto out;
> 
> Same here.

And there we explicitly set dev->postcopy_ufd = -1 before going to out
so it's sent as the error value.

> > +    }
> > +    /* TODO: Stash feature flags somewhere */
> > +out:
> > +    /* Return a ufd to the QEMU */
> > +    vmsg->fd_num = 1;
> > +    vmsg->fds[0] = dev->postcopy_ufd;

^^^ see that's where the -1's end up.
> >      return true; /* = send a reply */
> >  }

Dave

> >  
> > diff --git a/contrib/libvhost-user/libvhost-user.h b/contrib/libvhost-user/libvhost-user.h
> > index 3987ce643d..3e8efdd919 100644
> > --- a/contrib/libvhost-user/libvhost-user.h
> > +++ b/contrib/libvhost-user/libvhost-user.h
> > @@ -234,6 +234,9 @@ struct VuDev {
> >       * re-initialize */
> >      vu_panic_cb panic;
> >      const VuDevIface *iface;
> > +
> > +    /* Postcopy data */
> > +    int postcopy_ufd;
> >  };
> >  
> >  typedef struct VuVirtqElement {
> > -- 
> > 2.13.5
> > 
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK