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

[Kevin O'Connor]

On Fri, Apr 01, 2016 at 04:06:23PM +0100, Richard W.M. Jones wrote:
> On Fri, Apr 01, 2016 at 10:58:19AM -0400, Kevin O'Connor wrote:
> > On Fri, Apr 01, 2016 at 09:44:56AM +0100, Richard W.M. Jones wrote:
> > > On Fri, Apr 01, 2016 at 10:24:37AM +0200, Paolo Bonzini wrote:
> > > > On 01/04/2016 10:14, Richard W.M. Jones wrote:
> > > > > Found it: only CONFIG_MPTABLE=y was necessary.  It boots with:
> > > > > 
> > > > > # CONFIG_PIRTABLE is not set
> > > > > CONFIG_MPTABLE=y
> > > > > # CONFIG_SMBIOS is not set
> > > > > # CONFIG_ACPI is not set
> > > > 
> > > > If you add all three it should not give any slowdown and will provide
> > > > full hardware features to the kernel.  qboot does ACPI and PCI bus
> > > > assignment (it doesn't do SMBIOS because I got bored debugging it. :))
> > > 
> > > Enabling all 4 adds about 2ms.
> > 
> > CONFIG_SMBIOS and CONFIG_ACPI only control the legacy internal bios
> > tables.  One would disable CONFIG_FW_ROMFILE_LOAD to disable the newer
> > ACPI tables; there is no config option currently to disable the newer
> > smbios tables.
> > 
> > These config names are a bit misleading, so they probably should be
> > changed in seabios.
> > 
> > I didn't expect disabling the above options to do anything besides
> > reduce the size of the seabios binary, so I'm a bit suprised it saved
> > 2ms.  Did you have something on your qemu command-line to avoid
> > generating acpi/smbios tables?
> 
> The standard deviation of that number over 5 runs was 1ms, so I'm
> somewhat confident the 2ms is measuring something real.  In the grand
> scheme of things it doesn't matter at all.
> 
> I'm not using any qemu command line options related to acpi or smbios.

Okay - interesting.

Considering you have acpi=off in linux, you might want to try timing
with CONFIG_ACPI, CONFIG_SMBIOS, and CONFIG_FW_ROMFILE_LOAD all
disabled.  The acpi romfile loading stuff takes 4ms in my tests.

It's not clear to me that disabling acpi is a good plan, but it can't
hurt to time it.

-Kevin