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

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
>> So it's equivalent to enabling dirty tracking during live migration.  
>> In my mind, if the cost associated with hot plug is a fraction of the 
>> cost of live migration, we're in good shape.
>
> I don't see why.  Live migration is pretty expensive, but that doesn't 
> mean NIC hotplug should be.  Deployments where live migration isn't a 
> concern (for example, performance critical guests that use device 
> assignment) don't suffer the live migration penalty so they shouldn't 
> expect to see a gratuitous NIC hotplug penalty that is a fraction of 
> that.

Live migration is a feature that I expect will often be used in an 
automated fashion.  IOW, there will be some background process that 
automatically migrates a VM to improve overall utilization in a cloud.

NIC hotplug, on the other hand, will almost always be initiated by a 
user.  If there's a cost associated with it, at least it's driven by a 
user action so it's something a user can plan for.

>> Arguably, it's a much bigger problem for live migration.
>
> It is.  I once considered switching live migration to shadow pte dirty 
> bit tracking instead of write protection, but ept doesn't have dirty 
> bits, so this will only help a minority of deployments by the time it 
> reaches users.
>
> So vfork() is required, or in light of its man page and glowing 
> recommendations from the security people, we can mark guest memory as 
> shared on fork and use plain fork(), like we do for pre mmu notifier 
> kernels.

Indeed.

Regards,

Anthony Liguori