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-sta...@nongnu.org Reported-by: Gustavo Romero <gustavo.rom...@linaro.org> Signed-off-by: Richard Henderson <richard.hender...@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
