target-arm: Make page size a runtime setting

Rather than defining TARGET_PAGE_BITS to always be 10,
switch to using a value picked at runtime. This allows us
to use 4K pages for modern ARM CPUs (and in particular all
64-bit CPUs) without having to drop support for the old
ARMv5 CPUs which had 1K pages.
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Reviewed-by: NRichard Henderson <rth@twiddle.net>

target-arm/cpu.c

@@ -576,6 +576,7 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
     ARMCPU *cpu = ARM_CPU(dev);
     ARMCPUClass *acc = ARM_CPU_GET_CLASS(dev);
     CPUARMState *env = &cpu->env;
+    int pagebits;
 
     /* Some features automatically imply others: */
     if (arm_feature(env, ARM_FEATURE_V8)) {
@@ -631,6 +632,29 @@ static void arm_cpu_realizefn(DeviceState *dev, Error **errp)
         set_feature(env, ARM_FEATURE_THUMB_DSP);
     }
 
+    if (arm_feature(env, ARM_FEATURE_V7) &&
+        !arm_feature(env, ARM_FEATURE_M) &&
+        !arm_feature(env, ARM_FEATURE_MPU)) {
+        /* v7VMSA drops support for the old ARMv5 tiny pages, so we
+         * can use 4K pages.
+         */
+        pagebits = 12;
+    } else {
+        /* For CPUs which might have tiny 1K pages, or which have an
+         * MPU and might have small region sizes, stick with 1K pages.
+         */
+        pagebits = 10;
+    }
+    if (!set_preferred_target_page_bits(pagebits)) {
+        /* This can only ever happen for hotplugging a CPU, or if
+         * the board code incorrectly creates a CPU which it has
+         * promised via minimum_page_size that it will not.
+         */
+        error_setg(errp, "This CPU requires a smaller page size than the "
+                   "system is using");
+        return;
+    }
+
     if (cpu->reset_hivecs) {
             cpu->reset_sctlr |= (1 << 13);
     }

target-arm/cpu.h

@@ -1766,10 +1766,11 @@ bool write_cpustate_to_list(ARMCPU *cpu);
 #if defined(CONFIG_USER_ONLY)
 #define TARGET_PAGE_BITS 12
 #else
-/* The ARM MMU allows 1k pages.  */
-/* ??? Linux doesn't actually use these, and they're deprecated in recent
-   architecture revisions.  Maybe a configure option to disable them.  */
-#define TARGET_PAGE_BITS 10
+/* ARMv7 and later CPUs have 4K pages minimum, but ARMv5 and v6
+ * have to support 1K tiny pages.
+ */
+#define TARGET_PAGE_BITS_VARY
+#define TARGET_PAGE_BITS_MIN 10
 #endif
 
 #if defined(TARGET_AARCH64)