Will MSR instruction works immediately even during speculative execution?

MSR to TCO may execute speculatively, but its effect does NOT take effect during speculative execution. It only becomes architecturally visible when the instruction retires.

So there is no “fix” needed here this is intentional ARM behavior.

Even though the ARM ARM says special-purpose registers can be updated speculatively, that only means the core may track the new value internally during speculation. It does not mean the functional effect like MTE behavior changes via TCO is applied before retirement.

What "can occur speculatively" means here is mostly about the pipeline being allowed to execute the MSR early and carry the new value internally, not that the effect must be applied to later instructions during speculation.

In practice, implementations avoid letting a speculative TCO change affect subsequent speculative loads/stores. Those operations continue to behave as if the old TCO value is still active until the MSR retires. This is intentional.

The reason is to prevent speculative execution from changing memory checking behavior on a mispredicted path. If speculative loads/stores could observe a different TCO value, it could create observable side effects or even security issues. So the architecture allows the update to exist speculatively, but implementations are free to delay its functional impact until commit.

So yes, the update can be tracked speculatively, but its effect is typically not made visible to other speculative instructions by design.

Thanks. I know that the effect becomes architecturally visible only after the instruction retires. After the speculative execution is rolled back, the speculative update to TCO will also be rolled back. My question is about the speculative effect of the TCO update during speculative execution, not the architectural effect. Will the effect of speculatively updating to TCO be speculatively visible to subsequent load/store instructions that are speculatively executed? Given that such an update can occur speculatively, if its effect is not speculatively visible, what is the reason for such a design?