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
<kevin(a)koconnor.net> 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 <kevin(a)koconnor.net> 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 <kevin(a)koconnor.net> 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


--
Stefano Garzarella
Red Hat