Re: [PATCH RFC 07/20] migration: defer precopy policy to libxl

[Wei Liu <wei.liu2@citrix.com>]

On Thu, Mar 30, 2017 at 01:19:41AM -0400, Joshua Otto wrote:
> > 
> > > +};
> > > +
> > >  /* callbacks provided by xc_domain_save */
> > >  struct save_callbacks {
> > >      /* Called after expiration of checkpoint interval,
> > > @@ -46,6 +54,17 @@ struct save_callbacks {
> > >       */
> > >      int (*suspend)(void* data);
> > >  
> > > +    /* Called after every batch of page data sent during the precopy phase of a
> > > +     * live migration to ask the caller what to do next based on the current
> > > +     * state of the precopy migration.
> > > +     */
> > > +#define XGS_POLICY_ABORT          (-1) /* Abandon the migration entirely and
> > > +                                        * tidy up. */
> > > +#define XGS_POLICY_CONTINUE_PRECOPY 0  /* Remain in the precopy phase. */
> > > +#define XGS_POLICY_STOP_AND_COPY    1  /* Immediately suspend and transmit the
> > > +                                        * remaining dirty pages. */
> > > +    int (*precopy_policy)(struct precopy_stats stats, void *data);
> > 
> > Structures shouldn't be passed by value like this, as the compiler has
> > to do a lot of memcpy() work to make it happen.  You should pass by
> > const pointer, as (as far as I can tell), they are strictly read-only to
> > the implementation of this hook?
> 
> I chose to pass by value to make the IPC plumbing easier -
> libxl_save_msgs_gen.pl doesn't know what to do about pointers, and (not being
> the strongest Perl programmer...) I didn't want to volunteer to be the one to
> teach it.
> 
> Is the memcpy() really significant here?  If this were a tight loop, sure, but
> every invocation of the policy callback implies both a 4MB network transfer
> _and_ a synchronous RPC.

Ian, How can Joshua pass a pointer across RPC boundary to avoid excessive
copying?

Wei.

_______________________________________________
Xen-devel mailing list
Xen-devel@lists.xen.org
https://lists.xen.org/xen-devel