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

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 25 Mar 2020 10:16:36 +0100
Gerd Hoffmann <kraxel@redhat.com> wrote:

> > > 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  
> > I wonder what would be difference with hw-reduced acpi  
> 
> The kernel does a bunch of reads+writes at boot (roughly 20-30),
> mostly timer ports, so probably while initializing the pm_timer
> clocksource.  Then it stops talking to the acpi hardware,
> preferring kvmclock clocksouce I guess.
> 
> So I expect hw-reduced acpi wouldn't make much of a difference.
> Actually trying that isn't that high on my priority list.
> 
> There is bigger fish to fry, untangling the hw/i386/acpi-build.c
> monster for starters, so building with ACPI=y + PCI=n works ...

if you are after reducing start up time it may be better to leave alone
acpi-build.c with a bunch of legacy/compat stuff and create
microvm specific acpi file, and extract from acpi-build.c only common
bits that are necessary to produce tables (i.e. do something similar
hw/arm/virt-acpi-build.c)

PS:
side-note: guest kernel insist on finding PCI where it doesn't exists
[    0.301104] ACPI: bus type PCI registered
[    0.301717] acpiphp: ACPI Hot Plug PCI Controller Driver version: 0.5
[    0.302180] PCI: Fatal: No config space access function found

it's probably the first time x86 generic kernel sees machine without PCI,
and it probably could be fixed saving another 10-20ms


> 
> cheers,
>   Gerd
> 
>