Re: [PATCH 00/13] microvm: add acpi support

[Gerd Hoffmann]

On Mon, Mar 23, 2020 at 06:51:10AM -0400, Michael S. Tsirkin wrote:
> On Thu, Mar 19, 2020 at 09:01:04AM +0100, Gerd Hoffmann wrote:
> > I know that not supporting ACPI in microvm is intentional.  If you still
> > don't want ACPI this is perfectly fine, you can use the usual -no-acpi
> > switch to toggle ACPI support.
> > 
> > These are the advantages you are going to loose then:
> > 
> >   (1) virtio-mmio device discovery without command line hacks (tweaking
> >       the command line is a problem when not using direct kernel boot).
> >   (2) Better IO-APIC support, we can use IRQ lines 16-23.
> >   (3) ACPI power button (aka powerdown request) works.
> >   (4) machine poweroff (aka S5 state) works.
> 
> What is the cost though? How do boot times compare?

Well, acpi speeds up booting by one second because this delay ...

   [    0.275736] i8042: PNP: No PS/2 controller found.
   [    0.275736] i8042: Probing ports directly.
   [    1.315447] i8042: No controller found

... goes away, at least with standard distro kernels.  When building
your own you can probably compile out the driver somehow, even though
something seems to select SERIO_I8042 so trying to simply flip
CONFIG_SERIO_I8042 to 'n' in .config doesn't work.  And a runtime
switch seems to not be there either ...

So that ruined my plan to just time until the root filesystem is
mounted.  Decided to use the "i8042: PNP: No PS/2 controller found."
line instead for a simple test (just check the kernel log timestamps,
three runs each).  The ACPI initialization is already done at that
point, so it should be useful nevertheless.  Here we go:

Without acpi:
  0.277710
  0.278852
  0.279520

With acpi:
  0.283917
  0.284262
  0.284836

So the difference is less than 0.01 seconds on my box.

cheers,
  Gerd