Re: [PATCH v3 1/6] linux-user/aarch64: Choose SYNC as the preferred MTE mode

[Gustavo Romero]

On 2/6/24 11:52 PM, Richard Henderson wrote:
> The API does not generate an error for setting ASYNC | SYNC; that merely
> constrains the selection vs the per-cpu default.  For qemu linux-user,
> choose SYNC as the default.
>
> Cc: qemu-stable@nongnu.org
> Reported-by: Gustavo Romero <gustavo.romero@linaro.org>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>   linux-user/aarch64/target_prctl.h | 29 +++++++++++++++++------------
>   1 file changed, 17 insertions(+), 12 deletions(-)
>
> diff --git a/linux-user/aarch64/target_prctl.h 
> b/linux-user/aarch64/target_prctl.h
> index 5067e7d731..aa8e203c15 100644
> --- a/linux-user/aarch64/target_prctl.h
> +++ b/linux-user/aarch64/target_prctl.h
> @@ -173,21 +173,26 @@ static abi_long 
> do_prctl_set_tagged_addr_ctrl(CPUArchState *env, abi_long arg2)
>       env->tagged_addr_enable = arg2 & PR_TAGGED_ADDR_ENABLE;
>   
>       if (cpu_isar_feature(aa64_mte, cpu)) {
> -        switch (arg2 & PR_MTE_TCF_MASK) {
> -        case PR_MTE_TCF_NONE:
> -        case PR_MTE_TCF_SYNC:
> -        case PR_MTE_TCF_ASYNC:
> -            break;
> -        default:
> -            return -EINVAL;
> -        }
> -
>           /*
>            * Write PR_MTE_TCF to SCTLR_EL1[TCF0].
> -         * Note that the syscall values are consistent with hw.
> +         *
> +         * The kernel has a per-cpu configuration for the sysadmin,
> +         * /sys/devices/system/cpu/cpu<N>/mte_tcf_preferred,
> +         * which qemu does not implement.
> +         *
> +         * Because there is no performance difference between the modes, and
> +         * because SYNC is most useful for debugging MTE errors, choose SYNC
> +         * as the preferred mode.  With this preference, and the way the API
> +         * uses only two bits, there is no way for the program to select
> +         * ASYMM mode.
>            */
> -        env->cp15.sctlr_el[1] =
> -            deposit64(env->cp15.sctlr_el[1], 38, 2, arg2 >> PR_MTE_TCF_SHIFT);
> +        unsigned tcf = 0;
> +        if (arg2 & PR_MTE_TCF_SYNC) {
> +            tcf = 1;
> +        } else if (arg2 & PR_MTE_TCF_ASYNC) {
> +            tcf = 2;
> +        }
> +        env->cp15.sctlr_el[1] = deposit64(env->cp15.sctlr_el[1], 38, 2, tcf);
>   
>           /*
>            * Write PR_MTE_TAG to GCR_EL1[Exclude].

ok, so no ASYMM in QEMU user-mode, plus if both SYNC and ASYNC flags are
specified by the user SYNC is selected. Contrary to what happens by default
on Linux, because of the mte_tcf_preferred value, which is ASYNC, and the
final value selected is define by:

resolved_mte_tcf = (mte_ctrl & pref) ? pref : mte_ctrl; [0]

where pref is mte_tcf_preferred (CPU, the value set in sys /mte_tcf_preferred)
and mte_ctr comes from the process, i.e. is the value specified by the user in
the flags -- hence the default on Linux if both flags are specified is ASYNC,
not SYNC.

(just some notes for the records).

Thanks.


[0] 
https://github.com/torvalds/linux/blob/master/arch/arm64/kernel/mte.c#L180-L186