Dave Martin <Dave.Martin@xxxxxxx> writes:
> In preparation for optimising the way KVM manages switching the
> guest and host FPSIMD state, it is necessary to provide a means for
> code outside arch/arm64/kernel/fpsimd.c to restore the user trap
> configuration for SVE correctly for the current task.
>
> Rather than requiring external code to duplicate the maintenance
> explicitly, this patch wraps moves the trap maintenenace to
> fpsimd_bind_to_cpu(), since it is logically part of the work of
> associating the current task with the cpu.
>
> Because fpsimd_bind_to_cpu() is rather a cryptic name to publish
> alongside fpsimd_bind_state_to_cpu(), the former function is
> renamed to fpsimd_bind_task_to_cpu() to make its purpose more
> explicit.
>
> This patch makes appropriate changes to ensure that
> fpsimd_bind_task_to_cpu() is always called alongside
> task_fpsimd_load(), so that the trap maintenance continues to be
> done in every situation where it was done prior to this patch.
>
> As a side-effect, the metadata updates done by
> fpsimd_bind_task_to_cpu() now change from conditional to
> unconditional in the "already bound" case of sigreturn. This is
> harmless, and a couple of extra stores on this slow path will not
> impact performance. I consider this a reasonable price to pay for
> a slightly cleaner interface.
>
> Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> Acked-by: Marc Zyngier <marc.zyngier@xxxxxxx>
> Acked-by: Catalin Marinas <catalin.marinas@xxxxxxx>
In fact the comment I alluded to in 6/18 could be applied in this.
Reviewed-by: Alex Bennée <alex.bennee@xxxxxxxxxx>
> ---
> arch/arm64/kernel/fpsimd.c | 28 ++++++++++++++--------------
> 1 file changed, 14 insertions(+), 14 deletions(-)
>
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 1222491..ba9e7df 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -257,16 +257,6 @@ static void task_fpsimd_load(void)
> sve_vq_from_vl(current->thread.sve_vl) - 1);
> else
> fpsimd_load_state(¤t->thread.uw.fpsimd_state);
> -
> - if (system_supports_sve()) {
> - /* Toggle SVE trapping for userspace if needed */
> - if (test_thread_flag(TIF_SVE))
> - sve_user_enable();
> - else
> - sve_user_disable();
> -
> - /* Serialised by exception return to user */
> - }
> }
>
> /*
> @@ -991,7 +981,7 @@ void fpsimd_signal_preserve_current_state(void)
> * Associate current's FPSIMD context with this cpu
> * Preemption must be disabled when calling this function.
> */
> -static void fpsimd_bind_to_cpu(void)
> +static void fpsimd_bind_task_to_cpu(void)
> {
> struct fpsimd_last_state_struct *last =
> this_cpu_ptr(&fpsimd_last_state);
> @@ -999,6 +989,16 @@ static void fpsimd_bind_to_cpu(void)
> last->st = ¤t->thread.uw.fpsimd_state;
> last->sve_in_use = test_thread_flag(TIF_SVE);
> current->thread.fpsimd_cpu = smp_processor_id();
> +
> + if (system_supports_sve()) {
> + /* Toggle SVE trapping for userspace if needed */
> + if (test_thread_flag(TIF_SVE))
> + sve_user_enable();
> + else
> + sve_user_disable();
> +
> + /* Serialised by exception return to user */
> + }
> }
>
> /*
> @@ -1015,7 +1015,7 @@ void fpsimd_restore_current_state(void)
>
> if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE)) {
> task_fpsimd_load();
> - fpsimd_bind_to_cpu();
> + fpsimd_bind_task_to_cpu();
> }
>
> local_bh_enable();
> @@ -1038,9 +1038,9 @@ void fpsimd_update_current_state(struct user_fpsimd_state const *state)
> fpsimd_to_sve(current);
>
> task_fpsimd_load();
> + fpsimd_bind_task_to_cpu();
>
> - if (test_and_clear_thread_flag(TIF_FOREIGN_FPSTATE))
> - fpsimd_bind_to_cpu();
> + clear_thread_flag(TIF_FOREIGN_FPSTATE);
>
> local_bh_enable();
> }
--
Alex Bennée
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