hw/arm/boot: Drop existing dtb /psci node rather than retaining it

If we're using PSCI emulation, we add a /psci node to the device tree
we pass to the guest.  At the moment, if the dtb already has a /psci
node in it, we retain it, rather than replacing it. (This behaviour
was added in commit c39770cd in 2018.)

This is a problem if the existing node doesn't match our PSCI
emulation.  In particular, it might specify the wrong method (HVC vs
SMC), or wrong function IDs for cpu_suspend/cpu_off/etc, in which
case the guest will not get the behaviour it wants when it makes PSCI
calls.

An example of this is trying to boot the highbank or midway board
models using the device tree supplied in the kernel sources: this
device tree includes a /psci node that specifies function IDs that
don't match the (PSCI 0.2 compliant) IDs that QEMU uses.  The dtb
cpu_suspend function ID happens to match the PSCI 0.2 cpu_off ID, so
the guest hangs after booting when the kernel tries to idle the CPU
and instead it gets turned off.

Instead of retaining an existing /psci node, delete it entirely
and replace it with a node whose properties match QEMU's PSCI
emulation behaviour. This matches the way we handle /memory nodes,
where we also delete any existing nodes and write in ones that
match the way QEMU is going to behave.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Tested-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20220127154639.2090164-17-peter.maydell@linaro.org

hw/arm/boot.c

@@ -478,12 +478,13 @@ static void fdt_add_psci_node(void *fdt)
     }
 
     /*
-     * If /psci node is present in provided DTB, assume that no fixup
-     * is necessary and all PSCI configuration should be taken as-is
+     * A pre-existing /psci node might specify function ID values
+     * that don't match QEMU's PSCI implementation. Delete the whole
+     * node and put our own in instead.
      */
     rc = fdt_path_offset(fdt, "/psci");
     if (rc >= 0) {
-        return;
+        qemu_fdt_nop_node(fdt, "/psci");
     }
 
     qemu_fdt_add_subnode(fdt, "/psci");