Re: [Qemu-devel] [PATCH v3 0/7] Improvements and clean-ups related to -net

[Mark Cave-Ayland]

On 21/02/18 10:18, Thomas Huth wrote:

> "-net" is a legacy option that often causes confusion and
> misconfigurations for the users, since most people are not aware
> of the underlying "vlan" (i.e. hub) concept that is used for this
> parameter. The prefered way of configuring your network stack is
> to use "--netdev" instead, which gives you a clean 1:1 connection
> between your emulated guest hardware and the host network backend.
>
> However, there are two reasons why we could not completely deprecate
> "-net" yet:
>
> 1) Convenience:
> In some cases, it's more convenient to use "-net" instead of "--netdev",
> e.g. if you just want to have a "tap" network connection, it's faster
> to type "-net nic -net tap" instead of "--device e1000,netdev=n1
> --netdev tap,id=n1".
>
> 2) On-board NICs:
> Currently the "-net nic" parameter is the only way to configure on-
> board NICs on certain (embedded) machines via the nd_table[] array.
>
> So beside some generic clean-ups and removal of code that has been
> marked as deprecated since QEMU 2.10 already, this patch series intro-
> duces a new parameter "--nic" (in patch 7) which should be able to re-
> place "-net" in the long run completely: This new convenience parameter
> can be used to configure the default/on-board guest HW together with a
> host network backend in a very compact way. To configure a tap backend
> for the default NIC, you just have to type "--nic tap" here for example.

Hi Thomas,

This is a great improvement for configuring on-board NICs! I do have a 
couple of questions about your patchset based on personal experience:

1) Does the new -nic syntax support multiple on-board NICs? I remember 
seeing this on some of the ARM boards I was studying when trying to 
implement something similar for SPARC.

2) Is it possible to provide a convenient wrapper function to handle the 
logic related to determining whether a specified NIC is on-board or not?

For example take a look at 
https://git.qemu.org/?p=qemu.git;a=blob;f=hw/sparc64/sun4u.c;h=da28ab9413efdbe0bf0e1d3bf2b545577b83d88a;hb=a6e0344fa0e09413324835ae122c4cadd7890231#l596 
where we have a nice dance to check whether the device specified is 
on-board or not, and if it isn't to plug it in as an extra PCI device 
instead. The complication here is that nd_tables[] is populated by 
default (and also with -net nic -net user) but not when someone 
specifies a single card via -netdev.

From memory this was derived from the code in 
https://git.qemu.org/?p=qemu.git;a=blob;f=hw/arm/realview.c;h=87cd1e583cd20b6d8a2beeef1cba6977496d4477;hb=a6e0344fa0e09413324835ae122c4cadd7890231#l259 
if having another example helps?


ATB,

Mark.