Re: [Xen-devel] [PATCH 5/6] libxl: allow creation of domains with specified or random domid

["Durrant, Paul" <pdurrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Ian Jackson <ian.jackson@xxxxxxxxxx>
> Sent: 02 January 2020 17:27
> To: Durrant, Paul <pdurrant@xxxxxxxxxx>
> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; Wei Liu <wl@xxxxxxx>; Anthony Perard
> <anthony.perard@xxxxxxxxxx>
> Subject: Re: [PATCH 5/6] libxl: allow creation of domains with specified
> or random domid
> 
> Paul Durrant writes ("[PATCH 5/6] libxl: allow creation of domains with
> specified or random domid"):
> > This patch modifies do_domain_create() to use the value of domid that is
> > passed in. A new 'special value' - RANDOM_DOMID - is added into the API
> > and this, INVALID_DOMID or any valid domid is passed unmodified to
> > libxl__domain_make(). Any other invalid domid value will cause an error.
> >
> > If RANDOM_DOMID is passed in then libxl__domain_make() will use
> > libxl__random_bytes() to choose a domid. If the chosen value is not a
> > valid domid then this step will be repeated. Once a valid value is
> chosen
> > it will be passed to xc_domain_create() but if this fails with an errno
> > value of EEXIST, a new random value will be chosen and the operation
> will
> > be retried.
> 
> What is the use case for this ?
> 

The use-case is trying to make sure that, across a pool of hosts, the 
likelihood of creating two domains with the same domid is small. Because a 
transparent migration requires that domid is persisted, migrations are more 
likely to fail (due to a domid already being in use) if a sequential scheme is 
used.

> I think using this is hazardous if you ever destroy domains, because
> it will lead to reuse of domids in circumstances[1] where right now
> that can't happen.
> 
> [1] Fewer than ~2^16 creations per Xen boot.

Agreed, but is that actually a problem? A domain must be fully destroyed (i.e. 
all page refs dropped) before its id will become available for re-use. What are 
we actually trying to avoid by not immediately recycling? I'd certainly be open 
to adding some sort of retirement list for domids that would prevent re-use 
within a specified time, if that would help.

  Paul

> 
> Ian.

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