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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> |
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