Re: [Qemu-devel] [PATCH 00/16] Add a -netdev option

[Edgar E. Iglesias]

On Thu, Oct 08, 2009 at 04:37:41PM -0500, Anthony Liguori wrote:
> Stefan Weil wrote:
> > Hi Mark,
> >
> > today, qemu's ethernet devices are a fixed combination of
> > MAC (Media Access Controller) and PHY (Physical Layer).
> >
> > Real hardware is different: the controllers provide a MAC,
> > and many provide a PHY, too, but all controllers allow
> > one or even several external PHY(s).
> >
> > Most (or all?) controllers use the same standard interface
> > to address their PHY (internal or external), something
> > which is completely missing today in qemu.
> >
> > A better solution would use separate code for MAC and
> > PHY, so the controllers could share common code for the
> > PHY.
> >
> > Configuration of a different PHY should be possible,
> > so users can run a system emulation of different
> > MAC-PHY combinations.
> >
> > In addition to the PHY type, a PHY needs attributes
> > (link speed and link status).
> >   
> 
> But PHY doesn't really exist in the context of QEMU because virtualizing 
> at the ethernet level.

I think PHY's exist, at least with pure emulation.

Regarding the MII path I think you are right. But many if not most ethernet
controllers expose a way for software to directly communicate with PHY's
over MDIO. And software drivers usually read/write PHY registers to
check/change PHY related settings (speed, link status, duplex settings etc).

etraxfs_eth.c has a small dummy model of a PHY at the MDIO level. Enough to
fool software to think that's there is a one. Off-tree I've got at least 2
more controllers with emulated PHY's that are needed to please guest
software.

Cheers