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

From: "Jürgen Groß" <jgross@suse.com>
To: paul@xen.org, 'Julien Grall' <julien@xen.org>,
	xen-devel@lists.xenproject.org
Cc: 'Stefano Stabellini' <sstabellini@kernel.org>,
	'Wei Liu' <wl@xen.org>,
	'Andrew Cooper' <andrew.cooper3@citrix.com>,
	'Paul Durrant' <pdurrant@amazon.com>,
	'Ian Jackson' <ian.jackson@eu.citrix.com>,
	'George Dunlap' <george.dunlap@citrix.com>,
	'Jan Beulich' <jbeulich@suse.com>
Subject: Re: [PATCH v4] docs/designs: re-work the xenstore migration document...
Date: Tue, 28 Apr 2020 14:56:04 +0200	[thread overview]
Message-ID: <bb0a87e5-4112-107a-b15b-0edf1e616f87@suse.com> (raw)
In-Reply-To: <000c01d61d5b$00bc05c0$02341140$@xen.org>

On 28.04.20 14:46, Paul Durrant wrote:
>> -----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.

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.

Juergen