Re: [Qemu-devel] [PATCH 0/4] net-bridge: rootless bridge support for qemu

[Anthony Liguori <aliguori@us.ibm.com>]

Avi Kivity wrote:
> On 11/05/2009 07:20 PM, Arnd Bergmann wrote:
>> On Thursday 05 November 2009, Anthony Liguori wrote:
>>   
>>>> It'd still install the default helper you've provided and use it by
>>>> default, of course.
>>>>
>>>>        
>>> That's already how it behaves.  You can say -net
>>> bridge,helper=/usr/local/bin/my-helper
>>>
>>>      
>> How about abstracting it further and not making the helper depend on
>> bridge code. If we put the helper into netcf, we could make that
>> a more generic '-net netcf,helper=/usr/bin/netcf-helper' target,
>> with netcf doing the correct thing for the system configuration,
>> whether that is tap+bridge, tap+route, macvtap or something else
>> coming up. The helper would essentially become a black box for
>> providing a tap-like file descriptor with external connectivity.
>>    
>
> Helpers are really bad.  On launch, I find the fragile and hard to do 
> proper error handling with (but that's probably just me).  But the 
> real problem is at runtime, if you have a 16GB guest then you have to 
> write-protect 4M ptes and then kvm has to tear down or write protect 
> (not sure which mmu notifier is called) 4M shadow ptes.  Once that's 
> done, the guest will have to fault its way back; that's at least 4M 
> exits, around 10 seconds worth of cpu time to execute a couple of 
> syscalls.

FWIW, with large pages, it's only 8k exits which comes out to about 
16ms.  If nothing else, it's an argument for the important of 
transparent large pages.

-- 
Regards,

Anthony Liguori