target/arm: Default to 1GHz cntfrq for 'max' and new CPUs

In previous versions of the Arm architecture, the frequency of the
generic timers as reported in CNTFRQ_EL0 could be any IMPDEF value,
and for QEMU we picked 62.5MHz, giving a timer tick period of 16ns.
In Armv8.6, the architecture standardized this frequency to 1GHz.

Because there is no ID register feature field that indicates whether
a CPU is v8.6 or that it ought to have this counter frequency, we
implement this by changing our default CNTFRQ value for all CPUs,
with exceptions for backwards compatibility:

 * CPU types which we already implement will retain the old
   default value. None of these are v8.6 CPUs, so this is
   architecturally OK.
 * CPUs used in versioned machine types with a version of 9.0
   or earlier will retain the old default value.

The upshot is that the only CPU type that changes is 'max'; but any
new type we add in future (whether v8.6 or not) will also get the new
1GHz default.

It remains the case that the machine model can override the default
value via the 'cntfrq' QOM property (regardless of the CPU type).

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-id: 20240426122913.3427983-5-peter.maydell@linaro.org

2 files changed, 22 insertions, 0 deletions

diff --git a/target/arm/tcg/cpu32.c b/target/arm/tcg/cpu32.c
index b5a60682fa..bdd82d912a 100644
--- a/target/arm/tcg/cpu32.c
+++ b/target/arm/tcg/cpu32.c
@@ -457,6 +457,7 @@ static void cortex_a7_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_THUMB2EE);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_DUMMY_C15_REGS);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -505,6 +506,7 @@ static void cortex_a15_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_THUMB2EE);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_DUMMY_C15_REGS);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -696,6 +698,7 @@ static void cortex_r52_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_PMSA);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_AUXCR);
     cpu->midr = 0x411fd133; /* r1p3 */
@@ -924,6 +927,7 @@ static void arm_max_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
     set_feature(&cpu->env, ARM_FEATURE_EL3);
diff --git a/target/arm/tcg/cpu64.c b/target/arm/tcg/cpu64.c
index 443cffe3a8..da41a44f75 100644
--- a/target/arm/tcg/cpu64.c
+++ b/target/arm/tcg/cpu64.c
@@ -63,6 +63,7 @@ static void aarch64_a35_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -231,6 +232,7 @@ static void aarch64_a55_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -299,6 +301,7 @@ static void aarch64_a72_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -354,6 +357,7 @@ static void aarch64_a76_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -423,6 +427,7 @@ static void aarch64_a64fx_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
     set_feature(&cpu->env, ARM_FEATURE_EL3);
@@ -592,6 +597,7 @@ static void aarch64_neoverse_n1_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -663,6 +669,7 @@ static void aarch64_neoverse_v1_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -885,6 +892,7 @@ static void aarch64_a710_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -982,6 +990,7 @@ static void aarch64_neoverse_n2_initfn(Object *obj)
     set_feature(&cpu->env, ARM_FEATURE_V8);
     set_feature(&cpu->env, ARM_FEATURE_NEON);
     set_feature(&cpu->env, ARM_FEATURE_GENERIC_TIMER);
+    set_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
     set_feature(&cpu->env, ARM_FEATURE_AARCH64);
     set_feature(&cpu->env, ARM_FEATURE_CBAR_RO);
     set_feature(&cpu->env, ARM_FEATURE_EL2);
@@ -1078,6 +1087,15 @@ void aarch64_max_tcg_initfn(Object *obj)
     uint32_t u;
 
     /*
+     * Unset ARM_FEATURE_BACKCOMPAT_CNTFRQ, which we would otherwise default
+     * to because we started with aarch64_a57_initfn(). A 'max' CPU might
+     * be a v8.6-or-later one, in which case the cntfrq must be 1GHz; and
+     * because it is our "may change" CPU type we are OK with it not being
+     * backwards-compatible with how it worked in old QEMU.
+     */
+    unset_feature(&cpu->env, ARM_FEATURE_BACKCOMPAT_CNTFRQ);
+
+    /*
      * Reset MIDR so the guest doesn't mistake our 'max' CPU type for a real
      * one and try to apply errata workarounds or use impdef features we
      * don't provide.