Re: [Qemu-devel] SeaBIOS booting time optimization

From: Stefano Garzarella <sgarzare@redhat.com>
To: Gerd Hoffmann <kraxel@redhat.com>
Cc: Kevin OConnor <kevin@koconnor.net>,
	Samuel Ortiz <sameo@linux.intel.com>,
	seabios@seabios.org, Stefan Hajnoczi <stefanha@gmail.com>,
	Rob Bradford <robert.bradford@intel.com>,
	qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>,
	Paolo Bonzini <pbonzini@redhat.com>,
	stephend@silicom-usa.com, stefanb@linux.ibm.com
Subject: Re: [Qemu-devel] SeaBIOS booting time optimization
Date: Fri, 23 Nov 2018 12:18:13 +0100	[thread overview]
Message-ID: <CAGxU2F5-UUaJbwpKhTY1ZsRWVst=gM79nSjnZtZJVKDpime_Lw@mail.gmail.com> (raw)
In-Reply-To: <20181123062138.rdfaeaqq5uw43ey2@sirius.home.kraxel.org>

On Fri, Nov 23, 2018 at 7:21 AM Gerd Hoffmann <kraxel@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@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.

>
> > - 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)

Cheers,
Stefano

--
Stefano Garzarella
Red Hat