Re: [Qemu-devel] Why is SeaBIOS used with -kernel?

[Kevin O'Connor]

On Fri, Apr 01, 2016 at 07:41:31PM +0100, Richard W.M. Jones wrote:
> On Fri, Apr 01, 2016 at 11:35:40AM -0400, Kevin O'Connor wrote:
> > > +# general stuff
> > > +CONFIG_QEMU=y
> > > +CONFIG_ROM_SIZE=128
> > 
> > Why force a size of 128K - I would think 64K would be fine.
> 
> Agreed.  Setting this to =0 seems the best thing, and it does fit fine
> inside 64K.
> 
> > > +# no input, no boot menu
> > > +CONFIG_MOUSE=n
> > > +CONFIG_KEYBOARD=n
> > [...]
> > > +CONFIG_DRIVES=n
> > 
> > I would not recommended disabling CONFIG_MOUSE, CONFIG_KEYBOARD,
> > CONFIG_DRIVES - I only had those in my config so as to avoid having to
> > specify all the device drivers.  Ideally these would remain on and the
> > individual device drivers would be disabled.
> 
> We are always use this in a virtual appliance.  Interaction with the
> user is both impossible and undesirable.  It either boots or not, and
> the whole appliance is discarded in seconds.  We're always using
> -kernel with this SeaBIOS build, so probing drives is never needed.

Okay, but if it doesn't change the boot time, then it would be nicer
to use a standard rom for all boots.

I looked closer at your setup and it appears the SeaBIOS virtio-scsi
driver is very slow because it does a full search of all 256 possible
scsi targets.  This full scan takes a lot of time.  I put together a
quick patch (see below) to stop the scan early.  Gerd/Paulo, do you
know if what I've done is valid and/or if there is a better way we can
limit the virtio-scsi scan?

I also found a way to reduce the overhead of the "shadow ram" code a
little.  I have a patch (see below) for that as well.

Another consumer of time is ACPI table deployment.  I wonder if you
could get similar results by running QEMU with "-no-acpi"?

Beyond that, I think the only other big time consumers of the default
seabios is debug messages.  If so, then I think we can come up with a
way to limit these debug messages in SeaBIOS.

The SeaBIOS testing patches are at:

  https://github.com/KevinOConnor/seabios/tree/testing

Thanks,
-Kevin