arm64/fpsimd: Remove opportunistic freeing of SME state

When a task's SVE vector length (NSVL) is changed, and the task happens
to have SVCR.{SM,ZA}=={0,0}, vec_set_vector_length() opportunistically
frees the task's sme_state and clears TIF_SME.

The opportunistic freeing was added with no rationale in commit:

  d4d5be94a8787242 ("arm64/fpsimd: Ensure SME storage is allocated after SVE VL changes")

That commit fixed an unrelated problem where the task's sve_state was
freed while it could be used to store streaming mode register state,
where the fix was to re-allocate the task's sve_state.

There is no need to free and/or reallocate the task's sme_state when the
SVE vector length changes, and there is no need to clear TIF_SME. Given
the SME vector length (SVL) doesn't change, the task's sme_state remains
correctly sized.

Remove the unnecessary opportunistic freeing of the task's sme_state
when the task's SVE vector length is changed. The task's sme_state is
still freed when the SME vector length is changed.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Marc Zyngier <maz@kernel.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Will Deacon <will@kernel.org>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20250409164010.3480271-5-mark.rutland@arm.com
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
index 757445d72e5b731a0fa24eeb3287d55a46b7114b..128774015772a783da6342a4dbd2d335c21a87fd 100644 (file)
--- a/arch/arm64/kernel/fpsimd.c
+++ b/arch/arm64/kernel/fpsimd.c
@@ -868,19 +868,10 @@ int vec_set_vector_length(struct task_struct *task, enum vec_type type,
                task->thread.fp_type = FP_STATE_FPSIMD;
        }
 
-       if (system_supports_sme()) {
-               if (type == ARM64_VEC_SME ||
-                   !(task->thread.svcr & (SVCR_SM_MASK | SVCR_ZA_MASK))) {
-                       /*
-                        * We are changing the SME VL or weren't using
-                        * SME anyway, discard the state and force a
-                        * reallocation.
-                        */
-                       task->thread.svcr &= ~(SVCR_SM_MASK |
-                                              SVCR_ZA_MASK);
-                       clear_tsk_thread_flag(task, TIF_SME);
-                       free_sme = true;
-               }
+       if (system_supports_sme() && type == ARM64_VEC_SME) {
+               task->thread.svcr &= ~(SVCR_SM_MASK | SVCR_ZA_MASK);
+               clear_tsk_thread_flag(task, TIF_SME);
+               free_sme = true;
        }
 
        if (task == current)