RE: [PATCH v4] docs/designs: re-work the xenstore migration document...

[Paul Durrant <xadimgnik@xxxxxxxxx>]

> -----Original Message-----
> From: Jürgen Groß <jgross@xxxxxxxx>
> Sent: 28 April 2020 13:56
> To: paul@xxxxxxx; 'Julien Grall' <julien@xxxxxxx>; 
> xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: 'Paul Durrant' <pdurrant@xxxxxxxxxx>; 'Andrew Cooper' 
> <andrew.cooper3@xxxxxxxxxx>; 'George Dunlap'
> <george.dunlap@xxxxxxxxxx>; 'Ian Jackson' <ian.jackson@xxxxxxxxxxxxx>; 'Jan 
> Beulich'
> <jbeulich@xxxxxxxx>; 'Stefano Stabellini' <sstabellini@xxxxxxxxxx>; 'Wei Liu' 
> <wl@xxxxxxx>
> Subject: Re: [PATCH v4] docs/designs: re-work the xenstore migration 
> document...
> 
> On 28.04.20 14:46, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Julien Grall <julien@xxxxxxx>
> >> Sent: 28 April 2020 11:23
> >> To: Jürgen Groß <jgross@xxxxxxxx>; Paul Durrant <paul@xxxxxxx>; 
> >> xen-devel@xxxxxxxxxxxxxxxxxxxx
> >> Cc: Paul Durrant <pdurrant@xxxxxxxxxx>; Andrew Cooper 
> >> <andrew.cooper3@xxxxxxxxxx>; George Dunlap
> >> <george.dunlap@xxxxxxxxxx>; Ian Jackson <ian.jackson@xxxxxxxxxxxxx>; Jan 
> >> Beulich
> <jbeulich@xxxxxxxx>;
> >> Stefano Stabellini <sstabellini@xxxxxxxxxx>; Wei Liu <wl@xxxxxxx>
> >> Subject: Re: [PATCH v4] docs/designs: re-work the xenstore migration 
> >> document...
> >>
> >> Hi Juergen,
> >>
> >> On 28/04/2020 11:10, Jürgen Groß wrote:
> >>> On 28.04.20 11:05, Julien Grall wrote:
> >>>>> -where tx_id is the non-zero identifier values of an open transaction.
> >>>>> +| Field     | Description                                       |
> >>>>> +|-----------|---------------------------------------------------|
> >>>>> +| `domid`   | The domain-id that owns the shared page           |
> >>>>> +|           |                                                   |
> >>>>> +| `tdomid`  | The domain-id that `domid` acts on behalf of if   |
> >>>>> +|           | it has been subject to an SET_TARGET              |
> >>>>> +|           | operation [2] or DOMID_INVALID [3] otherwise      |
> >>>>> +|           |                                                   |
> >>>>> +| `flags`   | Must be zero                                      |
> >>>>> +|           |                                                   |
> >>>>> +| `evtchn`  | The port number of the interdomain channel used   |
> >>>>> +|           | by `domid` to communicate with xenstored          |
> >>>>> +|           |                                                   |
> >>>>> +| `mfn`     | The MFN of the shared page for a live update or   |
> >>>>> +|           | ~0 (i.e. all bits set) otherwise                  |
> >>>>> -### Protocol Extension
> >>>>> +Since the ABI guarantees that entry 1 in `domid`'s grant table will
> >>>>> always
> >>>>> +contain the GFN of the shared page, so for a live update `mfn` can
> >>>>> be used to
> >>>>> +give confidence that `domid` has not been re-cycled during the update.
> >>>>
> >>>> I am confused, there is no way a XenStored running in an Arm domain
> >>>> can map the MFN of the shared page. So how is this going to work out?
> >>>
> >>> I guess this will be a MFN for PV guests and a guest PFN else.
> >>
> >> If we use Xen terminology (xen/include/xen/mm.h), this would be a GFN.
> >>
> >
> > TBH I'm not sure a GFN would give much confidence about domain recycling as 
> > it would likely be the
> same for many domains, I think. We really need a UUID.
> 
> No, we need a per-host domain value, which is associated with a domain
> and which is unique during the life-time of the host.
> 
> E.g. a global counter, which is incremented at each domain creation and
> stored in struct domain.
> 

Can we just drop this and fall back on the fact that libxl now prevents domids 
from being recycled for 60s?

  Paul