[SeaBIOS] [PATCH v2] qemu: fast boot when linuxboot optionrom is used

[Stefano Garzarella]

On Thu, Nov 29, 2018 at 5:55 PM Kevin O'Connor <kevin at koconnor.net> wrote:
>
> On Thu, Nov 29, 2018 at 03:16:39PM +0100, Stefano Garzarella wrote:
> > On Wed, Nov 28, 2018 at 3:38 PM Kevin O'Connor <kevin at koconnor.net> wrote:
> > > It would be good to understand what the source of the delay is.  It
> > > might be possible to avoid the delay without adding a new config
> > > setting.  For example, the patch above will prevent the vga rom from
> > > being run, but it should be possible to accomplish the same with
> > > "-device VGA,romfile=".  The other change this patch makes is to skip
> > > device_hardware_setup() - is it known which items in that function add
> > > to the delay?
> >
> > Hi Kevin,
> > thanks for your advice.
> >
> > I tried to understand what are the sources of the delay, and I found
> > that most of the time is spent because of QEMU default devices (eg
> > e1000 ROM PCI, mouse, keyboard, etc)
> > At this point, (without the patch above) I disabled the default QEMU
> > devices (-nodefaults) and I reached 12.5 ms of SeaBIOS boot time
> > (compared to 22 ms, where 4 ms is taken by the loading of e1000 ROM).
> >
> > Other 2 ms is taken by enable_vga_console(), where simply comment out
> > the printf("SeaBIOS (version %s)\n", VERSION) I reached 10.7 ms of
> > boot time.
> > Do you think is acceptable to use dprintf(1, ...) instead of printf()?
> > Or maybe check if we found the VGA.
>
> Interesting.  I tracked down this printf delay - it's due to the
> save/restore of cpu state when thunking to 16bit mode.  (For every
> character displayed on the screen the code enters 16bit mode to invoke
> the vgabios and it saves/restores the cr0, gdt, fs, gs, a20, nmi
> states during that process.)  It's trivial to eliminate the calls when
> there is no vgabios though (see patch below).

Thanks, the patch works, but unfortunately, when I use qemu
-nographic, the  /etc/sercon-port is set to PORT_SERIAL1 (in
src/fw/paravirt.c:623), bypassing the patch.
Maybe in QEMU is better to set /etc/sercom-port to 0 when there is no
serial port, or when we want a fast boot.

>
> > Note: All these tests are done with CONFIG_DEBUG_LEVEL=0, using
> > CONFIG_DEBUG_LEVEL=1 the boot time grows up to 24 ms, maybe we should
> > put CONFIG_DEBUG_LEVEL=0 in the SeaBIOS configuration used in QEMU.
>
> I think the main seabios binary should have CONFIG_DEBUG_LEVEL=1 as it
> helps with debug reports.  I suppose an additional binary could be
> made for those looking for the fastest possible speed.  (The sole cost
> of the debugging is the additional hardware accesses that results from
> those debug messages.)

Okay, I'll discuss it in the QEMU mailing list.

>
> > > Also, what is the target boot time that is considered acceptable?
> >
> > I'm using qboot as a reference (6/7 ms). It is very minimal, so 10 ms
> > I think is considered acceptable for SeaBIOS
>
> Thanks,
> -Kevin
>
>
> --- a/src/output.c
> +++ b/src/output.c
> @@ -74,6 +74,9 @@ static struct putcinfo debuginfo = { debug_putc };
>  static void
>  screenc(char c)
>  {
> +    if (!MODESEGMENT && GET_IVT(0x10).segoff == FUNC16(entry_10).segoff)
> +        // No need to thunk to 16bit mode if vgabios is not present
> +        return;
>      struct bregs br;
>      memset(&br, 0, sizeof(br));
>      br.flags = F_IF;

Do you plan to commit this patch?

Thanks,
Stefano


--
Stefano Garzarella
Red Hat