Warner Losh <i...@bsdimp.com> writes: > On Tue, Dec 19, 2023, 1:55 PM Peter Maydell <peter.mayd...@linaro.org> > wrote: > >> On Tue, 19 Dec 2023 at 19:40, Fabiano Rosas <faro...@suse.de> wrote: >> > >> > Dave Blanchard <d...@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 >> >>
