target/arm: expose CPUID registers to userspace

A number of CPUID registers are exposed to userspace by modern Linux
kernels thanks to the "ARM64 CPU Feature Registers" ABI. For QEMU's
user-mode emulation we don't need to emulate the kernels trap but just
return the value the trap would have done. To avoid too much #ifdef
hackery we process ARMCPRegInfo with a new helper (modify_arm_cp_regs)
before defining the registers. The modify routine is driven by a
simple data structure which describes which bits are exported and
which are fixed.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20190205190224.2198-3-alex.bennee@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

target/arm/cpu.h

@@ -2464,6 +2464,27 @@ static inline void define_one_arm_cp_reg(ARMCPU *cpu, const ARMCPRegInfo *regs)
 }
 const ARMCPRegInfo *get_arm_cp_reginfo(GHashTable *cpregs, uint32_t encoded_cp);
 
+/*
+ * Definition of an ARM co-processor register as viewed from
+ * userspace. This is used for presenting sanitised versions of
+ * registers to userspace when emulating the Linux AArch64 CPU
+ * ID/feature ABI (advertised as HWCAP_CPUID).
+ */
+typedef struct ARMCPRegUserSpaceInfo {
+    /* Name of register */
+    const char *name;
+
+    /* Only some bits are exported to user space */
+    uint64_t exported_bits;
+
+    /* Fixed bits are applied after the mask */
+    uint64_t fixed_bits;
+} ARMCPRegUserSpaceInfo;
+
+#define REGUSERINFO_SENTINEL { .name = NULL }
+
+void modify_arm_cp_regs(ARMCPRegInfo *regs, const ARMCPRegUserSpaceInfo *mods);
+
 /* CPWriteFn that can be used to implement writes-ignored behaviour */
 void arm_cp_write_ignore(CPUARMState *env, const ARMCPRegInfo *ri,
                          uint64_t value);