You can start 'none' without "-serial null". Examples:
qemu-system-x86_64 -machine none
qemu-system-x86_64 -machine none -monitor stdio
qemu-system-x86_64 -machine none -nographic
qemu-system-x86_64 -machine none -monitor stdio -display none
Your command line "qemu-system-x86_64 -machine none -nographic -monitor
stdio" fails because "-nographic" says "please create a serial port
using stdio" but "-monitor stdio" tries to use stdio for something else.
You get the same message for any machine (eg "pc"), not just "none". If
what you wanted was "just don't create the graphical display" that's
"-display none" -- "-nographic" is a collection of things including both
'no display' and also 'default to creating a serial device to stdio' and
'default to creating a monitor muxed with that serial'.
** Changed in: qemu
Status: New => Invalid
--
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https://bugs.launchpad.net/bugs/1848231
Title:
serial/parallel character devices created for the none-machine
Status in QEMU:
Invalid
Bug description:
The none-machine can not be started unless using "-serial null":
qemu-system-x86_64 -machine none -nographic -monitor stdio
QEMU 3.1.1 monitor - type 'help' for more information
(qemu) qemu-system-x86_64: cannot use stdio by multiple character devices
qemu-system-x86_64: could not connect serial device to character backend
'stdio'
$
$ qemu-system-mips -machine none -nographic -serial null -monitor stdio
QEMU 4.1.50 monitor - type 'help' for more information
(qemu) info chardev
parallel0: filename=null
compat_monitor0: filename=stdio
serial0: filename=null
(qemu)
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