hw/arm/boot: Support setting psci-conduit based on guest EL

Currently we expect board code to set the psci-conduit property on
CPUs and ensure that secondary CPUs are created with the
start-powered-off property set to false, if the board wishes to use
QEMU's builtin PSCI emulation.  This worked OK for the virt board
where we first wanted to use it, because the virt board directly
creates its CPUs and is in a reasonable position to set those
properties.  For other boards which model real hardware and use a
separate SoC object, however, it is more awkward.  Most PSCI-using
boards just set the psci-conduit board unconditionally.

This was never strictly speaking correct (because you would not be
able to run EL3 guest firmware that itself provided the PSCI
interface, as the QEMU implementation would overrule it), but mostly
worked in practice because for non-PSCI SMC calls QEMU would emulate
the SMC instruction as normal (by trapping to guest EL3).  However,
we would like to make our PSCI emulation follow the part of the SMCC
specification that mandates that SMC calls with unknown function
identifiers return a failure code, which means that all SMC calls
will be handled by the PSCI code and the "emulate as normal" path
will no longer be taken.

We tried to implement that in commit 9fcd15b9193e81
("arm: tcg: Adhere to SMCCC 1.3 section 5.2"), but this
regressed attempts to run EL3 guest code on the affected boards:
 * mcimx6ul-evk, mcimx7d-sabre, orangepi, xlnx-zcu102
 * for the case only of EL3 code loaded via -kernel (and
   not via -bios or -pflash), virt and xlnx-versal-virt
so for the 7.0 release we reverted it (in commit 4825eaae4fdd56f).

This commit provides a mechanism that boards can use to arrange that
psci-conduit is set if running guest code at a low enough EL but not
if it would be running at the same EL that the conduit implies that
the QEMU PSCI implementation is using.  (Later commits will convert
individual board models to use this mechanism.)

We do this by moving the setting of the psci-conduit and
start-powered-off properties to arm_load_kernel().  Boards which want
to potentially use emulated PSCI must set a psci_conduit field in the
arm_boot_info struct to the type of conduit they want to use (SMC or
HVC); arm_load_kernel() will then set the CPUs up accordingly if it
is not going to start the guest code at the same or higher EL as the
fake QEMU firmware would be at.

Board/SoC code which uses this mechanism should no longer set the CPU
psci-conduit property directly.  It should only set the
start-powered-off property for secondaries if EL3 guest firmware
running bare metal expects that rather than the alternative "all CPUs
start executing the firmware at once".

Note that when calculating whether we are going to run guest
code at EL3, we ignore the setting of arm_boot_info::secure_board_setup,
which might cause us to run a stub bit of guest code at EL3 which
does some board-specific setup before dropping to EL2 or EL1 to
run the guest kernel. This is OK because only one board that
enables PSCI sets secure_board_setup (the highbank board), and
the stub code it writes will behave the same way whether the
one SMC call it makes is handled by "emulate the SMC" or by
"PSCI default returns an error code". So we can leave that stub
code in place until after we've changed the PSCI default behaviour;
at that point we will remove it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Tested-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Tested-by: Cédric Le Goater <clg@kaod.org>
Message-id: 20220127154639.2090164-4-peter.maydell@linaro.org

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