target/arm: Enable FEAT_CMOW for -cpu max

FEAT_CMOW introduces support for controlling cache maintenance
instructions executed in EL0/1 and is mandatory from Armv8.8.

On real hardware, the main use for this feature is to prevent processes
from invalidating or flushing cache lines for addresses they only have
read permission, which can impact the performance of other processes.

QEMU implements all cache instructions as NOPs, and, according to rule
[1], which states that generating any Permission fault when a cache
instruction is implemented as a NOP is implementation-defined, no
Permission fault is generated for any cache instruction when it lacks
read and write permissions.

QEMU does not model any cache topology, so the PoU and PoC are before
any cache, and rules [2] apply. These rules state that generating any
MMU fault for cache instructions in this topology is also
implementation-defined. Therefore, for FEAT_CMOW, we do not generate any
MMU faults either, instead, we only advertise it in the feature
register.

[1] Rule R_HGLYG of section D8.14.3, Arm ARM K.a.
[2] Rules R_MZTNR and R_DNZYL of section D8.14.3, Arm ARM K.a.

Signed-off-by: Gustavo Romero <gustavo.romero@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20241104142606.941638-1-gustavo.romero@linaro.org
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

docs/system/arm/emulation.rst

@@ -26,6 +26,7 @@ the following architecture extensions:
 - FEAT_BF16 (AArch64 BFloat16 instructions)
 - FEAT_BTI (Branch Target Identification)
 - FEAT_CCIDX (Extended cache index)
+- FEAT_CMOW (Control for cache maintenance permission)
 - FEAT_CRC32 (CRC32 instructions)
 - FEAT_Crypto (Cryptographic Extension)
 - FEAT_CSV2 (Cache speculation variant 2)

target/arm/cpu-features.h

@@ -802,6 +802,11 @@ static inline bool isar_feature_aa64_tidcp1(const ARMISARegisters *id)
     return FIELD_EX64(id->id_aa64mmfr1, ID_AA64MMFR1, TIDCP1) != 0;
 }
 
+static inline bool isar_feature_aa64_cmow(const ARMISARegisters *id)
+{
+    return FIELD_EX64(id->id_aa64mmfr1, ID_AA64MMFR1, CMOW) != 0;
+}
+
 static inline bool isar_feature_aa64_hafs(const ARMISARegisters *id)
 {
     return FIELD_EX64(id->id_aa64mmfr1, ID_AA64MMFR1, HAFDBS) != 0;

target/arm/cpu.h

@@ -1367,6 +1367,7 @@ void pmu_init(ARMCPU *cpu);
 #define SCTLR_EnIB    (1U << 30) /* v8.3, AArch64 only */
 #define SCTLR_EnIA    (1U << 31) /* v8.3, AArch64 only */
 #define SCTLR_DSSBS_32 (1U << 31) /* v8.5, AArch32 only */
+#define SCTLR_CMOW    (1ULL << 32) /* FEAT_CMOW */
 #define SCTLR_MSCEN   (1ULL << 33) /* FEAT_MOPS */
 #define SCTLR_BT0     (1ULL << 35) /* v8.5-BTI */
 #define SCTLR_BT1     (1ULL << 36) /* v8.5-BTI */

target/arm/helper.c

@@ -6229,6 +6229,11 @@ static void hcrx_write(CPUARMState *env, const ARMCPRegInfo *ri,
     if (cpu_isar_feature(aa64_nmi, cpu)) {
         valid_mask |= HCRX_TALLINT | HCRX_VINMI | HCRX_VFNMI;
     }
+    /* FEAT_CMOW adds CMOW */
+
+    if (cpu_isar_feature(aa64_cmow, cpu)) {
+        valid_mask |= HCRX_CMOW;
+    }
 
     /* Clear RES0 bits.  */
     env->cp15.hcrx_el2 = value & valid_mask;

target/arm/tcg/cpu64.c

@@ -1218,6 +1218,7 @@ void aarch64_max_tcg_initfn(Object *obj)
     t = FIELD_DP64(t, ID_AA64MMFR1, ETS, 2);      /* FEAT_ETS2 */
     t = FIELD_DP64(t, ID_AA64MMFR1, HCX, 1);      /* FEAT_HCX */
     t = FIELD_DP64(t, ID_AA64MMFR1, TIDCP1, 1);   /* FEAT_TIDCP1 */
+    t = FIELD_DP64(t, ID_AA64MMFR1, CMOW, 1);     /* FEAT_CMOW */
     cpu->isar.id_aa64mmfr1 = t;
 
     t = cpu->isar.id_aa64mmfr2;