| Message ID | 20240122163607.459769-3-peter.maydell@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | system: Fix handling of '-serial none -serial something' | expand |
On Mon, Jan 22, 2024 at 04:36:07PM +0000, Peter Maydell wrote: > The -serial option documentation is a bit brief about '-serial none' > and '-serial null'. In particular it's not very clear about the > difference between them, and it doesn't mention that it's up to > the machine model whether '-serial none' means "don't create the > serial port" or "don't wire the serial port up to anything". > > Expand on these points. > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org> > --- > qemu-options.hx | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) Reviewed-by: Daniel P. Berrangé <berrange@redhat.com> With regards, Daniel
Hi, On 22/1/24 17:36, Peter Maydell wrote: > The -serial option documentation is a bit brief about '-serial none' > and '-serial null'. In particular it's not very clear about the > difference between them, and it doesn't mention that it's up to > the machine model whether '-serial none' means "don't create the > serial port" or "don't wire the serial port up to anything". > > Expand on these points. > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org> > --- > qemu-options.hx | 14 +++++++++++--- > 1 file changed, 11 insertions(+), 3 deletions(-) > > diff --git a/qemu-options.hx b/qemu-options.hx > index ced82848637..d8c3fe91de1 100644 > --- a/qemu-options.hx > +++ b/qemu-options.hx > @@ -4129,7 +4129,8 @@ SRST > This option can be used several times to simulate up to 4 serial > ports. > > - Use ``-serial none`` to disable all serial ports. > + You can use ``-serial none`` to suppress the creation of default > + serial devices. > > Available character devices are: > > @@ -4151,10 +4152,17 @@ SRST > [Linux only] Pseudo TTY (a new PTY is automatically allocated) > > ``none`` > - No device is allocated. > + No device is allocated. Note that > for machine types which > + emulate systems where a serial device is always present in > + real hardware, this may be equivalent to the ``null`` option, > + in that the serial device is still present but all output > + is discarded. Should we deprecate this broken case, suggesting to use ``null`` instead? > For boards where the number of serial ports is > + truly variable, this suppresses the creation of the device. > > ``null`` > - void device > + A guest will see the UART or serial device as present in the > + machine, but all output is discarded, and there is no input. > + Conceptually equivalent to redirecting the output to ``/dev/null``. > > ``chardev:id`` > Use a named character device defined with the ``-chardev`` Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
On Mon, 22 Jan 2024 at 17:46, Philippe Mathieu-Daudé <philmd@linaro.org> wrote: > > Hi, > > On 22/1/24 17:36, Peter Maydell wrote: > > The -serial option documentation is a bit brief about '-serial none' > > and '-serial null'. In particular it's not very clear about the > > difference between them, and it doesn't mention that it's up to > > the machine model whether '-serial none' means "don't create the > > serial port" or "don't wire the serial port up to anything". > > > > Expand on these points. > > > > Signed-off-by: Peter Maydell <peter.maydell@linaro.org> > > --- > > qemu-options.hx | 14 +++++++++++--- > > 1 file changed, 11 insertions(+), 3 deletions(-) > > > > diff --git a/qemu-options.hx b/qemu-options.hx > > index ced82848637..d8c3fe91de1 100644 > > --- a/qemu-options.hx > > +++ b/qemu-options.hx > > @@ -4129,7 +4129,8 @@ SRST > > This option can be used several times to simulate up to 4 serial > > ports. > > > > - Use ``-serial none`` to disable all serial ports. > > + You can use ``-serial none`` to suppress the creation of default > > + serial devices. > > > > Available character devices are: > > > > @@ -4151,10 +4152,17 @@ SRST > > [Linux only] Pseudo TTY (a new PTY is automatically allocated) > > > > ``none`` > > - No device is allocated. > > + No device is allocated. Note that > > > for machine types which > > + emulate systems where a serial device is always present in > > + real hardware, this may be equivalent to the ``null`` option, > > + in that the serial device is still present but all output > > + is discarded. > > Should we deprecate this broken case, suggesting to use ``null`` > instead? It's machine specific. On systems with pluggable serial devices it makes sense to use '-serial none' to get rid of them entirely. On systems where the UARTs are hardwired into the board, having '-serial none' literally delete the UART device just breaks guests, which is why those boards make it behave like '-serial null'. But users should still be able to use '-serial none' to say "I don't really care about serial here". (This is why in commit 12051d82f004024d5d we made the chardev frontend functions cope with having a NULL backend, to avoid boards having to say "oh, serial_hd(n) is NULL, I must create a 'null' backend for it", which half of them didn't do.) thanks -- PMM
diff --git a/qemu-options.hx b/qemu-options.hx index ced82848637..d8c3fe91de1 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -4129,7 +4129,8 @@ SRST This option can be used several times to simulate up to 4 serial ports. - Use ``-serial none`` to disable all serial ports. + You can use ``-serial none`` to suppress the creation of default + serial devices. Available character devices are: @@ -4151,10 +4152,17 @@ SRST [Linux only] Pseudo TTY (a new PTY is automatically allocated) ``none`` - No device is allocated. + No device is allocated. Note that for machine types which + emulate systems where a serial device is always present in + real hardware, this may be equivalent to the ``null`` option, + in that the serial device is still present but all output + is discarded. For boards where the number of serial ports is + truly variable, this suppresses the creation of the device. ``null`` - void device + A guest will see the UART or serial device as present in the + machine, but all output is discarded, and there is no input. + Conceptually equivalent to redirecting the output to ``/dev/null``. ``chardev:id`` Use a named character device defined with the ``-chardev``
The -serial option documentation is a bit brief about '-serial none' and '-serial null'. In particular it's not very clear about the difference between them, and it doesn't mention that it's up to the machine model whether '-serial none' means "don't create the serial port" or "don't wire the serial port up to anything". Expand on these points. Signed-off-by: Peter Maydell <peter.maydell@linaro.org> --- qemu-options.hx | 14 +++++++++++--- 1 file changed, 11 insertions(+), 3 deletions(-)