target/arm: Make CP_ACCESS_TRAPs to AArch32 EL3 be Monitor traps

In system register access pseudocode the common pattern for
AArch32 registers with access traps to EL3 is:

at EL1 and EL2:
  if HaveEL(EL3) && !ELUsingAArch32(EL3) && (SCR_EL3.TERR == 1) then
     AArch64.AArch32SystemAccessTrap(EL3, 0x03);
  elsif HaveEL(EL3) && ELUsingAArch32(EL3) && (SCR.TERR == 1) then
     AArch32.TakeMonitorTrapException();
at EL3:
  if (PSTATE.M != M32_Monitor) && (SCR.TERR == 1) then
     AArch32.TakeMonitorTrapException();

(taking as an example the ERRIDR access pseudocode).

This implements the behaviour of (in this case) SCR.TERR that
"Accesses to the specified registers from modes other than Monitor
mode generate a Monitor Trap exception" and of SCR_EL3.TERR that
"Accesses of the specified Error Record registers at EL2 and EL1
are trapped to EL3, unless the instruction generates a higher
priority exception".

In QEMU we don't implement this pattern correctly in two ways:
 * in access_check_cp_reg() we turn the CP_ACCESS_TRAP_EL3 into
   an UNDEF, not a trap to Monitor mode
 * in the access functions, we check trap bits like SCR.TERR
   only when arm_current_el(env) < 3 -- this is correct for
   AArch64 EL3, but misses the "trap non-Monitor-mode execution
   at EL3 into Monitor mode" case for AArch32 EL3

In this commit we fix the first of these two issues, by
making access_check_cp_reg() handle CP_ACCESS_TRAP_EL3
as a Monitor trap. This is a kind of exception that we haven't
yet implemented(!), so we need a new EXCP_MON_TRAP for it.

This diverges from the pseudocode approach, where every access check
function explicitly checks for "if EL3 is AArch32" and takes a
monitor trap; if we wanted to be closer to the pseudocode we could
add a new CP_ACCESS_TRAP_MONITOR and make all the accessfns use it
when appropriate.  But because there are no non-standard cases in the
pseudocode (i.e.  where either it raises a Monitor trap that doesn't
correspond to an AArch64 SystemAccessTrap or where it raises a
SystemAccessTrap that doesn't correspond to a Monitor trap), handling
this all in one place seems less likely to result in future bugs
where we forgot again about this special case when writing an
accessor.

(The cc of stable here is because "hw/intc/arm_gicv3_cpuif: Don't
downgrade monitor traps for AArch32 EL3" which is also cc:stable
will implicitly use the new EXCP_MON_TRAP code path.)

Cc: qemu-stable@nongnu.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20250130182309.717346-6-peter.maydell@linaro.org

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