target/arm: Read debug-related ID registers from KVM

Now we have isar_feature test functions that look at fields in the
ID_AA64DFR0_EL1 and ID_DFR0 ID registers, add the code that reads
these register values from KVM so that the checks behave correctly
when we're using KVM.

No isar_feature function tests ID_AA64DFR1_EL1 or DBGDIDR yet, but we
add it to maintain the invariant that every field in the
ARMISARegisters struct is populated for a KVM CPU and can be relied
on.  This requirement isn't actually written down yet, so add a note
to the relevant comment.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200214175116.9164-13-peter.maydell@linaro.org

target/arm/cpu.h

@@ -853,6 +853,11 @@ struct ARMCPU {
      * prefix means a constant register.
      * Some of these registers are split out into a substructure that
      * is shared with the translators to control the ISA.
+     *
+     * Note that if you add an ID register to the ARMISARegisters struct
+     * you need to also update the 32-bit and 64-bit versions of the
+     * kvm_arm_get_host_cpu_features() function to correctly populate the
+     * field by reading the value from the KVM vCPU.
      */
     struct ARMISARegisters {
         uint32_t id_isar0;