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

[Kevin O'Connor]

On Fri, Nov 30, 2018 at 11:23:30AM +0100, Stefano Garzarella wrote:
> On Thu, Nov 29, 2018 at 5:55 PM Kevin O'Connor <[e-mail]> wrote:
> > 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.

You should be able to use "-device VGA,romfile=" instead.
> > --- 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?

Only if it's useful - does sercon make it not worthwhile?

-Kevin

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

[Stefano Garzarella]

Hi Kevin,

On Wed, Dec 5, 2018 at 6:00 PM Kevin O'Connor <[e-mail]> wrote:
> On Fri, Nov 30, 2018 at 11:23:30AM +0100, Stefano Garzarella wrote:
> > On Thu, Nov 29, 2018 at 5:55 PM Kevin O'Connor <[e-mail]> wrote:
> > > 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.
>
> You should be able to use "-device VGA,romfile=" instead.

My issue was releated to "-nographic" option. In this case if I
disable the VGA or if I use "-device VGA,romfile=", at the end of the
qemu_cfg_init() there are these lines that enable in any case the
sercom:

    // serial console
    u16 nogfx = 0;
    qemu_cfg_read_entry(&nogfx, QEMU_CFG_NOGRAPHIC, sizeof(nogfx));
    if (nogfx && !romfile_find("etc/sercon-port")
        && !romfile_find("vgaroms/sgabios.bin"))
        const_romfile_add_int("etc/sercon-port", PORT_SERIAL1);
}
> > > --- 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?
>
> Only if it's useful - does sercon make it not worthwhile?

Yes, I think it is useful. If I don't use "-nographic" it works as expected.

Thanks,
Stefano
> -Kevin