Re: [PATCH v2 1/2] arm64: add an MTE support check to the top of mte_thread_switch()

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Sep 14, 2021 at 02:54:28PM -0700, Peter Collingbourne wrote:
> This lets us avoid doing unnecessary work on hardware that does
> not support MTE, and will allow us to freely use MTE instructions
> in the code called by mte_thread_switch().
> 
> Signed-off-by: Peter Collingbourne <pcc@google.com>
> Link: https://linux-review.googlesource.com/id/I02fd000d1ef2c86c7d2952a7f099b254ec227a5d
> ---
>  arch/arm64/kernel/mte.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/arch/arm64/kernel/mte.c b/arch/arm64/kernel/mte.c
> index 9d314a3bad3b..8fc2d90eeb00 100644
> --- a/arch/arm64/kernel/mte.c
> +++ b/arch/arm64/kernel/mte.c
> @@ -199,6 +199,9 @@ void mte_thread_init_user(void)
>  
>  void mte_thread_switch(struct task_struct *next)
>  {
> +	if (!system_supports_mte())
> +		return;
> +
>  	mte_update_sctlr_user(next);

Ah, good point. While you are at this, could you please remove the
system_supports_mte() check from mte_check_tfsr_el1() and add it to
mte_check_tfsr_entry/exit? On the exit path, we unnecessarily do a
dsb+isb even when MTE is not present.

I guess we could also skip the tfsr_el1 check if
!system_uses_mte_async_mode() (well, I'd need to page in which TFSR bits
are set for uaccess).

Thanks.

-- 
Catalin

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