reset: Add RESET_TYPE_SNAPSHOT_LOAD

Some devices and machines need to handle the reset before a vmsave
snapshot is loaded differently -- the main user is the handling of
RNG seed information, which does not want to put a new RNG seed into
a ROM blob when we are doing a snapshot load.

Currently this kind of reset handling is supported only for:
 * TYPE_MACHINE reset methods, which take a ShutdownCause argument
 * reset functions registered with qemu_register_reset_nosnapshotload

To allow a three-phase-reset device to also distinguish "snapshot
load" reset from the normal kind, add a new ResetType
RESET_TYPE_SNAPSHOT_LOAD. All our existing reset methods ignore
the reset type, so we don't need to update any device code.

Add the enum type, and make qemu_devices_reset() use the
right reset type for the ShutdownCause it is passed. This
allows us to get rid of the device_reset_reason global we
were using to implement qemu_register_reset_nosnapshotload().

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Luc Michel <luc.michel@amd.com>
Message-id: 20240412160809.1260625-7-peter.maydell@linaro.org

1 files changed, 14 insertions, 3 deletions

diff --git a/docs/devel/reset.rst b/docs/devel/reset.rst
index 49baa1ea27..9746a4e8a0 100644
--- a/docs/devel/reset.rst
+++ b/docs/devel/reset.rst
@@ -27,9 +27,7 @@ instantly reset an object, without keeping it in reset state, just call
 ``resettable_reset()``. These functions take two parameters: a pointer to the
 object to reset and a reset type.
 
-Several types of reset will be supported. For now only cold reset is defined;
-others may be added later. The Resettable interface handles reset types with an
-enum:
+The Resettable interface handles reset types with an enum ``ResetType``:
 
 ``RESET_TYPE_COLD``
   Cold reset is supported by every resettable object. In QEMU, it means we reset
@@ -37,6 +35,19 @@ enum:
   from what is a real hardware cold reset. It differs from other resets (like
   warm or bus resets) which may keep certain parts untouched.
 
+``RESET_TYPE_SNAPSHOT_LOAD``
+  This is called for a reset which is being done to put the system into a
+  clean state prior to loading a snapshot. (This corresponds to a reset
+  with ``SHUTDOWN_CAUSE_SNAPSHOT_LOAD``.) Almost all devices should treat
+  this the same as ``RESET_TYPE_COLD``. The main exception is devices which
+  have some non-deterministic state they want to reinitialize to a different
+  value on each cold reset, such as RNG seed information, and which they
+  must not reinitialize on a snapshot-load reset.
+
+Devices which implement reset methods must treat any unknown ``ResetType``
+as equivalent to ``RESET_TYPE_COLD``; this will reduce the amount of
+existing code we need to change if we add more types in future.
+
 Calling ``resettable_reset()`` is equivalent to calling
 ``resettable_assert_reset()`` then ``resettable_release_reset()``. It is
 possible to interleave multiple calls to these three functions. There may