Re: [RFC v2] xSplice design

[Ross Lagerwall <ross.lagerwall@citrix.com>]

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.

-- 
Ross Lagerwall