Re: [Qemu-devel] [PATCH v2 4/9] vhost: alloc shareable log

["Marc-André Lureau" <mlureau@redhat.com>]

Hi

----- Original Message -----
> * Marc-André Lureau (marcandre.lureau@redhat.com) wrote:
> > If the backend is of type VHOST_BACKEND_TYPE_USER, allocate
> > shareable memory.
> > 
> > Note: vhost_log_get() can use a global "vhost_log" that can be shared by
> > several vhost devices. We may want instead a common shareable log and a
> > common non-shareable one.
> > 
> > Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
> > ---
> >  hw/virtio/vhost.c         | 38 +++++++++++++++++++++++++++++++-------
> >  include/hw/virtio/vhost.h |  3 ++-
> >  2 files changed, 33 insertions(+), 8 deletions(-)
> > 
> > diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> > index 2712c6f..862e786 100644
> > --- a/hw/virtio/vhost.c
> > +++ b/hw/virtio/vhost.c
> > @@ -18,6 +18,7 @@
> >  #include "qemu/atomic.h"
> >  #include "qemu/range.h"
> >  #include "qemu/error-report.h"
> > +#include "qemu/memfd.h"
> >  #include <linux/vhost.h>
> >  #include "exec/address-spaces.h"
> >  #include "hw/virtio/virtio-bus.h"
> > @@ -286,20 +287,34 @@ static uint64_t vhost_get_log_size(struct vhost_dev
> > *dev)
> >      }
> >      return log_size;
> >  }
> > -static struct vhost_log *vhost_log_alloc(uint64_t size)
> > +
> > +static struct vhost_log *vhost_log_alloc(uint64_t size, bool share)
> >  {
> > -    struct vhost_log *log = g_malloc0(sizeof *log + size *
> > sizeof(*(log->log)));
> > +    struct vhost_log *log;
> > +    uint64_t logsize = size * sizeof(*(log->log));
> > +    int fd = -1;
> > +
> > +    log = g_new0(struct vhost_log, 1);
> > +    if (share) {
> > +        log->log = qemu_memfd_alloc("vhost-log", logsize,
> > +                                    F_SEAL_GROW|F_SEAL_SHRINK|F_SEAL_SEAL,
> > &fd);
> > +        memset(log->log, 0, logsize);
> 
> qemu_memfd_alloc can return NULL can't it - so that needs checking?
> 
> > +    } else {
> > +        log->log = g_malloc0(logsize);
> 
> I know the old code also used g_malloc0, but if the log isn't 'small'
> then g_try_malloc0 is possibly safer and properly return errors
> if it can't be allocated.

Yeah, I agree it's better to check for the return value here (as you pointed out, I followed the existing pattern).

Maybe we are just screwed if it happens, live migration shouldn't succeed if it can't be done properly imho.

What's your take on this Michael?

cheers