Re: [Qemu-devel] [PATCH] CMOS file support

[Stefan Weil]

Am 17.09.2010 08:42, schrieb Mathias Krause:
> Am 16.09.2010 18:49, Stefan Weil schrieb:
> > The intention of this patch is ok. Loading CMOS with initial data
> > is needed. I just want to add two questions / remarks how the
> > implementation might be improved.
> >
> > Are there use cases where having a smaller CMOS size is better?
> > For example, when I want to emulate a system with 128 byte CMOS?
> > The size of CMOS could be derived from the size of the data
> > or specified by an additional parameter.
>
> I cannot image cases where smaller sizes would be a benefit. The saved
> disk space is negligible and you can always pad your CMOS configuration
> uo to 256 bytes by filling the gap with zero bytes. If the system just
> accesses the first 128 bytes the additional 128 zero bytes shouldn't hurt.

The benefit of smaller sizes is not saving RAM but precision
of the emulation.

A BIOS or an operating system might be designed to support
CMOS  with 128 bytes or 256 bytes and try guessing the size
by probing (many systems do something like this for RAM or
other memory types).

Testing the support of the small CMOS will be impossible
if the emulation always emulates 256 bytes. Filling the
additional 128 bytes with zero bytes won't help because
the system will access them successfully although writes
should fail.


> > Using QEMU's block devices instead of a simple file would be
> > more consistent with the rest of QEMU and allow reading the
> > CMOS data not only from a file but also from an URL or other
> > sources.
>
> Thanks for the hint. Since this is my first contribution to the project
> I'm not that familiar with the code. Looking at other places, e.g. how
> the -kernel option gets handled, I just see FILE everywhere. Can you
> give me some pointers how to use this interface?
>
> Thanks for the review!
>
>
> Mathias