[SeaBIOS] [Qemu-devel] SeaBIOS booting time optimization

[Kevin O'Connor]

On Fri, Nov 23, 2018 at 12:18:13PM +0100, Stefano Garzarella wrote:
> On Fri, Nov 23, 2018 at 7:21 AM Gerd Hoffmann <kraxel at redhat.com> wrote:
> > On Thu, Nov 22, 2018 at 04:13:38PM +0100, Stefano Garzarella wrote:
> > > On Thu, Nov 22, 2018 at 12:51 PM Gerd Hoffmann <kraxel at redhat.com> wrote:
> > > > On Thu, Nov 22, 2018 at 12:08:55PM +0100, Stefano Garzarella wrote:
> > > > > Hi,
> > > > > I continued to investigate how to reduce the boot time with SeaBIOS
> > > > > and QEMU when it used with linuxboot_dma.bin (-kernel parameter).
> > > > > I reached ~12ms with a SeaBIOS configuration (attached) where I
> > > > > disabled debug output, all Hardware support (except SMM & MTRRs) and I
> > > > > applied a small patch to disable VGA setup and console (attached).
> > > >
> > > > Is there any difference to "qemu -vga none" ?
> > >
> > > Using both (qemu -vga none, and my patch) we are around 10.8 ms.
> > > Note: using only the patch, Linux is still able to initialize and use the VGA.
> >
> > But do you want linux use the vga console if you care about boot times?
> > I'd expect virtio-console would be fastest in that case.
> 
> I agree with you, but if we will implement a QEMU fastboot feature in
> SeaBIOS, we are sure that it works with Linux kernel if someone uses
> "qemu -kernel" and wants also use a VGA.

It should be possible to use "-device VGA,romfile=" to enable VGA
without an optionrom.

> > > - QEMU -vga none + SeaBIOS config (CONFIG_DEBUG_LEVEL=0, disable all
> > > HW support except
> > > SMM & MTRRs) + Stephen's TPM patch
> > >  qemu_init_end: 43.675803
> > >  fw_start: 43.865178 (+0.189375)
> > >  fw_do_boot: 58.093161 (+14.227983)
> > >  linux_start_boot: 59.490308 (+1.397147)
> > >  linux_start_user: 556.782354 (+497.292046)
> > >
> > > - QEMU -vga none + SeaBIOS config (CONFIG_DEBUG_LEVEL=0, disable all
> > > HW support except
> > > SMM & MTRRs, CONFIG_DISABLE_VGA=y) + Stephen's TPM patch
> > >  qemu_init_end: 42.387412
> > >  fw_start: 42.579257 (+0.191845)
> > >  fw_do_boot: 53.381517 (+10.802260)
> > >  linux_start_boot: 54.848643 (+1.467126)
> > >  linux_start_user: 498.517050 (+443.668407)
> >
> > Interesting that CONFIG_DISABLE_VGA=y makes a noticable difference even
> > without vga hardware being preset.  And not only in seabios but also for
> > the linux kernel.
> >
> > Do you know why?
> 
> In SeaBIOS I think the reasons are:
> - vgarom_setup() scan all PCI devices to find a VGA
> - enable_vga_console() invokes an int10 without check if there is a
> VGA (maybe here we can add a check)

Where is CONFIG_DISABLE_VGA set - it's not a standard SeaBIOS config
option?

In my experience, the only meaningful delays are the result of
accessing hardware.  Neither of the above two items would result in
additional hardware accesses, so I don't think they would be the cause
of an additional delay.

-Kevin