Re: [PATCH] clarify SHUTDOWN_suspend additional argument

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 08/05/14 11:11, Ian Campbell wrote:
> On Wed, 2014-05-07 at 14:05 +0100, Stefano Stabellini wrote:
>> SCHEDOP_shutdown has a third argument that is unused on HVM and ARM
>> guests. Those guests pass 0 instead. Clarify the behaviour in the
>> hypercall description.
>>
>> Signed-off-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
> Acked-by: Ian Campbell <ian.campbell@citrix.com>
>
> But:
>> diff --git a/xen/include/public/sched.h b/xen/include/public/sched.h
>> index a30b11d..c170556 100644
>> --- a/xen/include/public/sched.h
>> +++ b/xen/include/public/sched.h
>> @@ -77,8 +77,9 @@
>>   * @arg == pointer to sched_shutdown_t structure.
>>   *
>>   * If the sched_shutdown_t reason is SHUTDOWN_suspend then this
>> - * hypercall takes an additional extra argument which should be the
>> - * MFN of the guest's start_info_t.
>> + * hypercall takes an additional extra argument which should be:
>> + *  - the MFN of the guest's start_info_t for x86 PV guests;
>> + *  - 0 for x86 HVM guests and arm and arm64 guests.
> Is this strictly true or is the argument ignored for those guests?
> (Requiring it to be zero doesn't conflict with that hence the ack)

Having recently played with code here, I believe it is as follows.

* Xen does absolutely nothing with the value whatsoever.
* PV Migration *must* check the value (and indeed converts it to a pfn
for transit)
* HVM guests (including arm for these purposes) don't have easy access
to the register, and just transmit the Xen architectural state blob as-is.

Therefore, the only requirement I can see is that PV guests must point
it at the start_info_t mfn.

~Andrew

>
>>   *
>>   * In addition, which reason is SHUTDOWN_suspend this hypercall
>>   * returns 1 if suspend was cancelled or the domain was merely
>
>
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