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

[Gleb Natapov]

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.

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

> 
> 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.