Re: [Qemu-devel] [PATCH v3 11/29] vhost+postcopy: Transmit 'listen' to client

[Dr. David Alan Gilbert]

* Peter Xu (address@hidden) wrote:
> On Mon, Mar 05, 2018 at 05:42:42PM +0000, Dr. David Alan Gilbert wrote:
> > * Peter Xu (address@hidden) wrote:
> > > On Fri, Feb 16, 2018 at 01:16:07PM +0000, Dr. David Alan Gilbert (git) 
> > > wrote:
> > > 
> > > [...]
> > > 
> > > >  typedef struct VuVirtqElement {
> > > > diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
> > > > index 621543e654..bdec9ec0e8 100644
> > > > --- a/docs/interop/vhost-user.txt
> > > > +++ b/docs/interop/vhost-user.txt
> > > > @@ -682,6 +682,12 @@ Master message types
> > > >        the slave must open a userfaultfd for later use.
> > > >        Note that at this stage the migration is still in precopy mode.
> > > >  
> > > > + * VHOST_USER_POSTCOPY_LISTEN
> > > > +      Id: 27
> > > > +      Master payload: N/A
> > > > +
> > > > +      Master advises slave that a transition to postcopy mode has 
> > > > happened.
> > > 
> > > Could we add something to explain why this listen needs to be
> > > broadcasted to clients?  Since I failed to find it out quickly
> > > myself. :(
> > 
> > I've changed this to:
> > 
> >  * VHOST_USER_POSTCOPY_LISTEN
> >       Id: 29
> >       Master payload: N/A
> > 
> >       Master advises slave that a transition to postcopy mode has happened.
> >       The slave must ensure that shared memory is registered with 
> > userfaultfd
> >       to cause faulting of non-present pages.
> 
> But shouldn't this be assured by the SET_MEM_TABLE call?

Yes, it is the set_mem_table that does the register - but it only
registers it with userfaultfd if it's received a 'listen' notification,
otherwise it assumes it's a normal precopy migration.

> Sorry for being not that familiar with vhost-user protocol... but
> what's the correct order of these commands?
> 
>   POSTCOPY_ADVISE
>   POSTCOPY_LISTEN
>   SET_MEM_TABLE

Right.

Dave

> ?  Thanks,
> 
> > 
> >       This is always sent sometime after a VHOST_USER_POSTCOPY_ADVISE, and
> >       thus only when VHOST_USER_PROTOCOL_F_PAGEFAULT is supported.
> > 
> > Dave
> > 
> > > -- 
> > > Peter Xu
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
> 
> -- 
> Peter Xu
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK