Re: [PATCH 16/16] xen/events: use the FIFO-based ABI if available

[Boris Ostrovsky <boris.ostrovsky@oracle.com>]

On 10/16/2013 05:46 AM, David Vrabel wrote:
> On 15/10/13 21:39, Boris Ostrovsky wrote:
>> On 10/15/2013 02:58 PM, David Vrabel wrote:
>>> On 14/10/13 20:30, Boris Ostrovsky wrote:
>>>> On 10/08/2013 08:49 AM, David Vrabel wrote:
>>>>> @@ -1636,7 +1637,13 @@ void xen_callback_vector(void) {}
>>>>>       void __init xen_init_IRQ(void)
>>>>>     {
>>>>> -    xen_evtchn_2l_init();
>>>>> +    int ret;
>>>>> +
>>>>> +    ret = xen_evtchn_fifo_init();
>>>>> +    if (ret < 0) {
>>>>> +        printk(KERN_INFO "xen: falling back to n-level event
>>>>> channels");
>>>>> +        xen_evtchn_2l_init();
>>>>> +    }
>>>> Should we provide users with ability to choose which mechanism to use?
>>>> Is there any advantage in staying with 2-level? Stability, I guess,
>>>> would be one.
>>> If someone can demonstrate a use case where 2-level is better then we
>>> could consider an option.  I don't think we want options for new
>>> software features just because they might be buggy.
>> I think we should always try to have a way to force old behavior when a
>> new feature is introduced. If a user discovers a bug we don't want them
>> to wait for a fix when a simpler solution is possible. I can see that
>> having this option in the kernel may be a bit too much but is there at
>> least an option to force 2-level in the hypervisor?
> The majority of the risk in this series is the refactoring patches and
> not the new ABI code so an option to disable it wouldn't really help.
>
> Also, if we are not confidant that a feature is production ready that we
> need knobs to turn it off then we shouldn't merge it.

I disagree. If this were the case then hardware wouldn't have chicken 
bits and there wouldn't exist half of "no*" options to kernel, for example.

We are introducing a rewrite of a critical component of the system. It 
has bugs (by definition) and there should be a way to fall back to an 
implementation that presumably has fewer bugs.

-boris