Re: [Qemu-devel] Re: [PATCH-updated] qemu/net: add raw backend

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Oct 14, 2009 at 10:58:22AM -0500, Anthony Liguori wrote:
> Jamie Lokier wrote:
>> Anthony Liguori wrote:
>>   
>>> Or Gerlitz wrote:
>>>     
>>>> Add raw network backend option which uses a packet socket to provide
>>>> raw networking access. Once the socket is opened it's bound to a
>>>> provided host interface, such that packets received on the interface
>>>> are delivered to the VM and packets sent by the VM are sent to the
>>>> interface.
>>>>
>>>> This is functionally similar to the existing pcap network
>>>> backend, with the same advantages and problems.
>>>> Differences from pcap:
>>>> - can get an open socket from the monitor,
>>>>  which allows running without NET_ADMIN priviledges
>>>> - support iovec sends with writev, saving one data copy
>>>> - one less dependency on an external library
>>>> - we have access to the underlying file descriptor
>>>>  which makes it possible to connect to vhost net
>>>> - don't support polling all interfaces, always bind to a specific one
>>>>        
>>> Networking is probably the area in qemu that users most frequently  
>>> stumble with.  The most common problems are:
>>>
>>> 1) slirp does not behave how they think it should (icmp doesn't work, 
>>> guest isn't accessable from host)
>>> 2) it's difficult to figure out which backend behaves the way they 
>>> want (socket vs. vde vs. tap)
>>> 3) when they figure out they need tap, tap is difficult to setup     
>>
>> Worse, tap is impossible to setup properly with things like
>> network-manager.
>>   
>
> This is being fixed.

I keep hearing this. But why wait? Some users can be helped
by raw today. Those that can't will have to wait for a better bridge
or spend time setting it up properly.

>
>> I suspect user expectations are quite commonly:
>>
>>    - guest<->host networking works
>>    - guest<->host's network works, directly or through host NAT
>>    - guest IP address is either private (inside the host)
>>      or on the same network as the host, according to some switch.
>>
>> Imho, there is only one right place to fix this, and it's by adding a
>> feature to the host.  Either modifying host packet socket, or
>> modifying the tap+bridge combination.
>>
>> Neither tap nor pcap/raw works particularly well except in static IP
>> configurations, and qemu cannot realistically work around the
>> host configuration difficulties.
>>   
>
> The fact that network manager does work well with bridged interfaces is  
> a network manager bug.

Network manager was just one example.

> It's getting fixed so in the near future, tap  
> will satisfy all of these requirements.
>> It'd be great if vhost_net doesn't have the configuration problems of
>> tap or pcap/raw.  If it does have the same problems, it's a natural
>> place to fix them.  I haven't looked at vhost_net yet.
>>
>> -- Jamie
>>   
>
> Regards,
>
> Anthony Liguori