Re: Qemu resets terminal to crazy defaults

[Fabiano Rosas <farosas@suse.de>]

Warner Losh <imp@bsdimp.com> writes:

> On Tue, Dec 19, 2023, 1:55 PM Peter Maydell <peter.maydell@linaro.org>
> wrote:
>
>> On Tue, 19 Dec 2023 at 19:40, Fabiano Rosas <farosas@suse.de> wrote:
>> >
>> > Dave Blanchard <dave@killthe.net> writes:
>> >
>> > > Hello all, can you please help me to understand what Qemu is doing
>> here?
>> > >
>> > > When connecting to the guest for example using a serial/tcp/telnet
>> link, some kind of code is being immediately transmitted over the link
>> which screws up my Xterm terminal settings, including changing the text
>> cursor shape and most notably, disabling wraparound of long lines, so that
>> they get truncated at the edge of the window instead.
>> > >
>> > > Can this behavior be disabled by command line, and if not, what is the
>> code doing exactly so I can know where to disable it? I tried disabling all
>> calls to tcsetattr() but that had no effect.
>>
>> > I looked into the automatic margins issue a long time ago and I seem to
>> > remember it was caused by the firmware (SeaBIOS) configuring the
>> > terminal and QEMU just never returning it to the original state. I
>> > eventually gave up trying to fix it because I was having trouble finding
>> > a reliable point in QEMU shutdown sequence to enable the capability
>> > back. Nowadays I just run 'tput smam' after quitting QEMU.
>>
>> To check whether this is happening because of the BIOS (or other
>> guest code) vs QEMU itself, you can try running QEMU in a configuration
>> where it doesn't run any BIOS code. One I happen to know offhand
>> is an arm one:
>>
>>    qemu-system-aarch64 -M virt -serial stdio
>>
>> This won't print anything, because we haven't loaded any guest
>> code at all and there's no default BIOS on this machine type.
>> (The emulated CPU is sat in effectively a tight loop taking
>> exceptions.) If that messes up the terminal settings, then it's
>> likely being done by something inside QEMU. If it doesn't, then
>> it sounds like as you say it'll be because of the SeaBIOS
>> firmware writing stuff to the terminal.
>>
>> (There might be a way to run the x86 PC machine without it
>> running a BIOS, for a similar test, but I don't know if there
>> is or how to do it off the top of my head.)
>>

I tried using an empty bios file. I see with 'info registers' that the
vcpu is spinning. After quitting QEMU, the terminal state is unchanged:

$ dd if=/dev/zero of=dummy-bios.bin count=256 bs=1k
$ qemu-system-x86_64 -nographic -bios ./dummy-bios.bin
$ <line wrap preserved>

With SeaBIOS, the issue manifests:

$ qemu-system-x86_64 -nographic
SeaBIOS (version rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org)
<bunch of boot and iPXE messages>
...
$ <line wrap disabled>

>> I do know that QEMU doesn't clean up things the guest does
>> to the terminal, because for instance if you have a serial
>> terminal and the guest puts it into "emit boldface/bright",
>> that doesn't go back to normal non-bold text when QEMU exits.
>> (It would be nice if it did do that...)
>>
>
> It would be nice indeed. Trouble is quarrying the state beforehand to know
> what to reset by random software producing effectively random bytes..
>

Maybe we could focus on the more annoying/obvious state? The line wrap
issue is a very salient one, specially since QEMU command lines
themselves tend to take more than one line.

> ESC c
>
> is the reset sequence as well...but that's likely too big a hammer.
>
> Warner
>
> thanks
>> -- PMM
>>
>>