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

[Paul Durrant <xadimgnik@gmail.com>]

> -----Original Message-----
> From: Julien Grall <julien@xen.org>
> Sent: 28 April 2020 11:23
> To: Jürgen Groß <jgross@suse.com>; Paul Durrant <paul@xen.org>; xen-devel@lists.xenproject.org
> Cc: Paul Durrant <pdurrant@amazon.com>; Andrew Cooper <andrew.cooper3@citrix.com>; George Dunlap
> <george.dunlap@citrix.com>; Ian Jackson <ian.jackson@eu.citrix.com>; Jan Beulich <jbeulich@suse.com>;
> Stefano Stabellini <sstabellini@kernel.org>; Wei Liu <wl@xen.org>
> 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.

> >
> >>
> >> [...]
> >>
> >>> -START_DOMAIN_TRANSACTION    <domid>|<transid>|
> >>> +    0       1       2       3    octet
> >>> ++-------+-------+-------+-------+
> >>> +| conn-id                       |
> >>> ++-------------------------------+
> >>> +| tx-id                         |
> >>> ++---------------+---------------+
> >>> +| path-len      | value-len     |
> >>> ++---------------+---------------+
> >>> +| access        | perm-count    |
> >>> ++---------------+---------------+
> >>> +| perm1                         |
> >>> ++-------------------------------+
> >>> +...
> >>> ++-------------------------------+
> >>> +| permN                         |
> >>> ++---------------+---------------+
> >>> +| path
> >>> +...
> >>> +| value
> >>> +...
> >>> +```
> >>> +
> >>> +
> >>> +| Field        | Description                                    |
> >>> +|--------------|------------------------------------------------|
> >>> +| `conn-id`    | If this value is non-zero then this record     |
> >>> +|              | related to a pending transaction               |
> >>
> >> If conn-id is 0, how do you know the owner of the node?
> >
> > The first permission entry's domid is the owner.
> 
> I think this should be spell out in the specification.
> 

In xenstore.txt, there is a reference to https://wiki.xen.org/wiki/XenBus

This explains things reasonably well; I'll reference it here and add some words too.

  Paul