[clearview-discuss] 2008/242 Data Fast-Path for Softmac

[Cathy Zhou]

Garrett D'Amore wrote:
> Cathy Zhou wrote:
>> Erik Nordmark wrote:
>>> Sebastien Roy wrote:
>>>
>>>> Interfaces
>>>> ==========
>>>>
>>>>     - mac_fastpath_disable()/mac_fastpath_enable() (Consolidation 
>>>> Private)
>>> Why can't those be project private? Presumably no MAC driver other 
>>> that softmac should ever use this.
>>>
>> These are not used by softmac driver. They are used by mac clients. At 
>> this point only the vnic driver will need to call these functions. But 
>> it could be used by any MAC clients that don't work with fast-path.
> 
> So the mac clients need to be aware of fast path?  That sounds rather 
> unfortunate -- given that the ultimate goal is to expose the mac client 
> API someday.  It seems like the details of this should be an 
> implementation detail, that mac clients don't have to worry about.
> 
Unfortunately, because softmac is not able to know whether some specific 
function can work with the fast-path data-model or not, mac clients have to 
specify that information.

But I think you made a valid point that these two functions should be 
classified as "Project Private" today, as all other mac client interfaces are 
all "Project Private" today (defined in PSARC 2004/571).

I will make these changes to the case material.

> Perhaps more clearly, I don't want MAC clients to fail to function 
> correctly if they don't enable this behavior.
> 
> I'd rather have an explicit "enable" that is used by fast-path aware 
> clients (probably only the IP stuff), and let all other clients 
> automatically start without fast path enabled.  That way if someone 
> writes a mac client that doesn't know about fastpath, it will at least 
> function properly.
> 
Note that VNIC is currently the only mac client that does not work with 
fastpath. Aggregations (for example) can co-exist with fast-path just fine. 
Further, even we assume IP is the only mac client which works with fast-path, 
we have to have some kind of mac client interface to tell the difference: 
assume that IP is plumbed on a legacy device, and fast-path is enabled 
explicitly, then some other mac client starts to use this legacy device and it 
does not work with fast-path, we still need to disable fast-path dynamically. 
More important, when that mac client goes away, and IP is now the only one 
user of this legacy device, then we should fall back to the fast-path again.

Thanks
- Cathy

> (Yes, I still believe that this whole fastpath mechanism is essentially 
> a performance hack to work around a few crufty drivers that really 
> should be updated to be pure GLDv3 drivers instead of moving this cruft 
> into the framework.  But on the assumption that we're going to move 
> forward with this, lets at least limit the visibility of this API to 
> minimize its architectural impact on the system.)
> 
>     -- Garrett
> 
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