[PATCH] qemu-options: Deprecate -old-param command line option

[Peter Maydell]

The '-old-param' command line option is specific to Arm targets; it
is very briefly documented as "old param mode".  What this option
actually does is change the behaviour when directly booting a guest
kernel, so that command line arguments are passed to the kernel using
the extremely old "param_struct" ABI, rather than the newer ATAGS or
even newer DTB mechanisms.

This support was added back in 2007 to support an old vendor kernel
on the akita/terrier board types:
 https://mail.gnu.org/archive/html/qemu-devel/2007-07/msg00344.html
Even then, it was an out-of-date mechanism from the kernel's
point of view -- the kernel has had a comment since 2001 marking
it as deprecated. As of mid-2024, the kernel only retained
param_struct support for the RiscPC and Footbridge platforms:
 
https://lore.kernel.org/linux-arm-kernel/2831c5a6-cfbf-4fe0-b51c-0396e5b0aeb7@app.fastmail.com/

None of the board types QEMU supports need param_struct support;
mark this option as deprecated.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 docs/about/deprecated.rst | 13 +++++++++++++
 system/vl.c               |  1 +
 2 files changed, 14 insertions(+)

diff --git a/docs/about/deprecated.rst b/docs/about/deprecated.rst
index 29de49351d1..a4c7d642236 100644
--- a/docs/about/deprecated.rst
+++ b/docs/about/deprecated.rst
@@ -68,6 +68,19 @@ configurations (e.g. -smp drawers=1,books=1,clusters=1 for 
x86 PC machine) is
 marked deprecated since 9.0, users have to ensure that all the topology members
 described with -smp are supported by the target machine.
 
+``-old-param`` option for booting Arm kernels via param_struct (since 10.0)
+'''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
+
+The ``-old-param`` command line option is specific to Arm targets:
+it is used when directly booting a guest kernel to pass it the
+command line and other information via the old ``param_struct`` ABI,
+rather than the newer ATAGS or DTB mechanisms. This option was only
+ever needed to support ancient kernels on some old board types
+like the ``akita`` or ``terrier``; it has been deprecated in the
+kernel since 2001. None of the board types QEMU supports need
+``param_struct`` support, so this option has been deprecated and will
+be removed in a future QEMU version.
+
 User-mode emulator command line arguments
 -----------------------------------------
 
diff --git a/system/vl.c b/system/vl.c
index c5678267182..32ee1ce7041 100644
--- a/system/vl.c
+++ b/system/vl.c
@@ -3438,6 +3438,7 @@ void qemu_init(int argc, char **argv)
                 nb_prom_envs++;
                 break;
             case QEMU_OPTION_old_param:
+                warn_report("-old-param is deprecated");
                 old_param = 1;
                 break;
             case QEMU_OPTION_rtc:
-- 
2.34.1