Re: [RFC v2] xSplice design

[Martin Pohlack <mpohlack@amazon.com>]

On 29.10.2015 17:55, Ross Lagerwall wrote:
> On 10/27/2015 12:05 PM, Ross Lagerwall wrote:
>> On 06/12/2015 12:39 PM, Martin Pohlack wrote:
>>> On 15.05.2015 21:44, Konrad Rzeszutek Wilk wrote:
>>> [...]
>>>> ## Hypercalls
>>>>
>>>> We will employ the sub operations of the system management hypercall
>>>> (sysctl).
>>>> There are to be four sub-operations:
>>>>
>>>>   * upload the payloads.
>>>>   * listing of payloads summary uploaded and their state.
>>>>   * getting an particular payload summary and its state.
>>>>   * command to apply, delete, or revert the payload.
>>>>
>>>> The patching is asynchronous therefore the caller is responsible
>>>> to verify that it has been applied properly by retrieving the summary
>>>> of it
>>>> and verifying that there are no error codes associated with the payload.
>>>>
>>>> We **MUST** make it asynchronous due to the nature of patching: it
>>>> requires
>>>> every physical CPU to be lock-step with each other. The patching
>>>> mechanism
>>>> while an implementation detail, is not an short operation and as such
>>>> the design **MUST** assume it will be an long-running operation.
>>>
>>> I am not convinced yet, that you need an asynchronous approach here.
>>>
>>> The experience from our prototype suggests that hotpatching itself is
>>> not an expensive operation.  It can usually be completed well below 1ms
>>> with the most expensive part being getting the hypervisor to a quiet
>>> state.
>>>
>>
>> FWIW, my current implementation (which is almost certainly not optimal)
>> tested on a 72 CPU machine takes about 3ms, whether idle or fully loaded.
>>
> 
> Let me correct that: it takes 60 μs to 100 μs to synchronize and apply 
> the patch (on the same hardware) when synchronous console logging is 
> turned off.

The interesting (and very rare) case is if other CPUs are busy in Xen
already, for example, with memory scrubbing or other long-running
activities.  Those are hard to interrupt and delay patching activity.

Having multiple guests in a reboot-loop / being restarted all the time
might help triggering this case.

Martin
Amazon Development Center Germany GmbH
Krausenstr. 38
10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B
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