Re: [Qemu-devel] Question about qemu firmware configuration (fw_cfg) device

[Anthony Liguori]

On 07/19/2010 09:53 AM, Gleb Natapov wrote:
> On Mon, Jul 19, 2010 at 09:45:58AM -0500, Anthony Liguori wrote:
>    
> > On 07/19/2010 02:33 AM, Gleb Natapov wrote:
> >      
> > > On Mon, Jul 19, 2010 at 08:28:02AM +0100, Richard W.M. Jones wrote:
> > >        
> > > > On Mon, Jul 19, 2010 at 09:23:56AM +0300, Gleb Natapov wrote:
> > > >          
> > > > > That what I am warring about too. If we are adding device we have to be
> > > > > sure such device can actually exist on real hw too otherwise we may have
> > > > > problems later.
> > > > >            
> > > > I don't understand why the constraints of real h/w have anything to do
> > > > with this.  Can you explain?
> > > >
> > > >          
> > > Each time we do something not architectural it cause us troubles later.
> > > So constraints of real h/w is our constrains to.
> > >        
> > Your constraints are purely artificial.
> >
> >      
> What is artificial about it? Each time we break them we safer.
>    

Just because something doesn't fit as an ISA or PCI device doesn't mean 
it can't exist in real life.  There are plenty of one-off devices with 
odd interfaces.

> > There are plenty of places that something like fw_cfg could live and
> > still do DMA.  It can directly hang off of the Southbridge.  It
> > doesn't necessary need to be connected to the ISA/LPC buses.
> >      
> Examples of real HW?

The IBM IMM, HP ILO, or Intel iAMT modules.  They basically play an 
identical role to fw_cfg.

>   And I am not against something that does DMA,
> but that is not what proposed patch does. It provides magic io
> instruction that CPU calls and when instruction completes memory is
> updated. This is nothing like DMA.

Isn't this exactly what the interface for PCI DMA looks like since 
there's no standard DMA implementation?

>   Of course it is possible to add
> proper DMA interface to fw_cfg, but should we do it for such a small
> gain?
>    

I think an ad-hoc DMA interface is perfectly reasonable to do.  I agree 
that adding a more generic DMA interface is overkill.

Regards,

Anthony Liguori

> > Buses exist to multiplex I/O devices because of limited wiring space
> > on motherboards.  There's no reason we need to constrain ourselves
> > to minimize the number of virtual wires we emulate.
> >
> > Regards,
> >
> > Anthony Liguori
> >      
> --
>                         Gleb.
>