Commit d6728f301d7e ("add serial console support") causes double-printing

[Omar Sandoval]

Hello,

I found that QEMU v2.11.0 double prints to the console when running the
Arch Linux installation ISO. I bisected this to seabios commit
d6728f301d7e ("add serial console support").

The expected output is this:

┌────────────────────────────────────────────────────────────────────┐
    │                             Arch Linux                             │
    ├────────────────────────────────────────────────────────────────────┤
    │ Boot Arch Linux (x86_64)                                           │
    │ Boot existing OS                                                   │
    │ Run Memtest86+ (RAM test)                                          │
    │ Hardware Information (HDT)                                         │
    │ Reboot                                                             │
    │ Power Off                                                          │
    │                                                                    │
    └────────────────────────────────────────────────────────────────────┘

But instead I get the following mess:

┌┌────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
    │                             Arch Linux                             ││
    ├├────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┤┤
    │  BBoooott  AArrcchh  LLiinnuuxx  ((xx8866__6644))                                                                                      ││
    │  BBoooott  eexxiissttiinngg  OOSS                                                                                                      ││
    │  RRuunn  MMeemmtteesstt8866++  ((RRAAMM  tteesstt))                                                                                    ││
    │  HHaarrddwwaarree  IInnffoorrmmaattiioonn  ((HHDDTT))                                                                                  ││
    │  RReebboooott                                                                                                                          ││
    │  PPoowweerr  OOffff                                                                ││
    │                                                                                                                                        ││
    └└────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘┘

To reproduce this, download the latest ISO from https://www.archlinux.org/download/, then run

qemu-system-x86_64 -nodefaults -nographic -serial mon:stdio -cpu kvm64 -enable-kvm -boot d -no-reboot -cdrom archlinux-2018.01.01-x86_64.iso

Thanks!
Omar

[Gerd Hoffmann]

Hi,
> But instead I get the following mess:
> │  BBoooott  AArrcchh  LLiinnuuxx  ((xx8866__6644))

Seems both seabios and boot loader print to the serial line.
> qemu-system-x86_64 -nodefaults -nographic -serial mon:stdio -cpu kvm64 -enable-kvm -boot d -no-reboot -cdrom archlinux-2018.01.01-x86_64.iso

Try "-display none" instead of "-nographic".

"-nographic" is a shortcut which does several things, one of them is
setting the "-machine graphics=off" guest hint, which in turn enables
the serial console in seabios.

cheers,
  Gerd

[Omar Sandoval]

On Thu, Jan 11, 2018 at 10:07:03AM +0100, Gerd Hoffmann wrote:
> Hi,
> > But instead I get the following mess:
> > │  BBoooott  AArrcchh  LLiinnuuxx  ((xx8866__6644))
>
> Seems both seabios and boot loader print to the serial line.
> > qemu-system-x86_64 -nodefaults -nographic -serial mon:stdio -cpu kvm64 -enable-kvm -boot d -no-reboot -cdrom archlinux-2018.01.01-x86_64.iso
>
> Try "-display none" instead of "-nographic".
>
> "-nographic" is a shortcut which does several things, one of them is
> setting the "-machine graphics=off" guest hint, which in turn enables
> the serial console in seabios.

Indeed, that fixes it. Thanks!