target-arm: Don't mention PMU in debug feature register

Suppress the ID_AA64DFR0_EL1 PMUVer field, even if the CPU specific
value claims that it exists. QEMU doesn't currently implement it,
and not advertising it prevents the guest from trying to use it
and getting UNDEFs on unimplemented registers.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NPeter Crosthwaite <peter.crosthwaite@xilinx.com>
---
This is arguably a hack, but otherwise Linux tries to prod
half a dozen PMU sysregs.

target-arm/helper.c

@@ -2036,7 +2036,12 @@ void register_cp_regs_for_features(ARMCPU *cpu)
             { .name = "ID_AA64DFR0_EL1", .state = ARM_CP_STATE_AA64,
               .opc0 = 3, .opc1 = 0, .crn = 0, .crm = 5, .opc2 = 0,
               .access = PL1_R, .type = ARM_CP_CONST,
-              .resetvalue = cpu->id_aa64dfr0 },
+              /* We mask out the PMUVer field, beacuse we don't currently
+               * implement the PMU. Not advertising it prevents the guest
+               * from trying to use it and getting UNDEFs on registers we
+               * don't implement.
+               */
+              .resetvalue = cpu->id_aa64dfr0 & ~0xf00 },
             { .name = "ID_AA64DFR1_EL1", .state = ARM_CP_STATE_AA64,
               .opc0 = 3, .opc1 = 0, .crn = 0, .crm = 5, .opc2 = 1,
               .access = PL1_R, .type = ARM_CP_CONST,