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Fri, 11 Sep 2026 08:41:58 -0700 (PDT) From: Jens Axboe To: io-uring@vger.kernel.org Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, tglx@kernel.org, mingo@redhat.com, peterz@infradead.org, Jens Axboe Subject: [PATCH 03/15] arm64: implement thread identity handoff Date: Fri, 11 Sep 2026 09:40:53 -0600 Message-ID: <20260911154148.644489-4-axboe@kernel.dk> X-Mailer: git-send-email 2.55.0 In-Reply-To: <20260911154148.644489-1-axboe@kernel.dk> References: <20260911154148.644489-1-axboe@kernel.dk> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Implement the arch_thread_handoff_*() hooks and select ARCH_HAS_THREAD_HANDOFF. The source is inside a syscall, so only the FPSIMD view of the vector registers needs to move. Tasks with SME streaming mode or ZA enabled, compat tasks, tasks with a guarded control stack and tasks trapping counter-timer access are refused. Signed-off-by: Jens Axboe --- arch/arm64/Kconfig | 1 + arch/arm64/kernel/process.c | 109 ++++++++++++++++++++++++++++++++++++ 2 files changed, 110 insertions(+) diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig index b5a51b0ef944..a919570c80cd 100644 --- a/arch/arm64/Kconfig +++ b/arch/arm64/Kconfig @@ -46,6 +46,7 @@ config ARM64 select ARCH_HAS_PTE_SPECIAL select ARCH_HAS_HW_PTE_YOUNG select ARCH_HAS_SETUP_DMA_OPS + select ARCH_HAS_THREAD_HANDOFF select ARCH_HAS_SET_DIRECT_MAP select ARCH_HAS_SET_MEMORY select ARCH_HAS_FORCE_DMA_UNENCRYPTED diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c index 581f80e9b9b7..e669a2717106 100644 --- a/arch/arm64/kernel/process.c +++ b/arch/arm64/kernel/process.c @@ -11,6 +11,7 @@ #include #include #include +#include #include #include #include @@ -1003,3 +1004,111 @@ int set_tsc_mode(unsigned int val) return do_set_tsc_mode(val); } + +#ifdef CONFIG_THREAD_HANDOFF +/* + * Thread identity handoff. The source is inside a syscall, so only the FPSIMD + * view of the vector registers needs to move. SME state is refused. + */ +static bool thread_handoff_task_ok(struct task_struct *tsk) +{ + if (is_compat_thread(task_thread_info(tsk))) + return false; + /* the GCS is per-thread and would have to move along */ + if (task_gcs_el0_enabled(tsk)) + return false; + /* counter-timer trapping doesn't move, see update_cntkctl_el1() */ + if (test_tsk_thread_flag(tsk, TIF_TSC_SIGSEGV)) + return false; + return true; +} + +bool arch_thread_handoff_allowed(struct task_struct *tsk) +{ + return thread_handoff_task_ok(tsk); +} + +bool arch_thread_handoff_compatible(struct task_struct *src, + struct task_struct *dst) +{ + return thread_handoff_task_ok(dst); +} + +/* sync the live user state, fpsimd_syscall_enter() guarantees FPSIMD format */ +bool arch_thread_handoff_prepare(void) +{ + struct thread_struct *thread = ¤t->thread; + + fpsimd_preserve_current_state(); + tls_preserve_current_state(); + if (system_supports_poe()) + thread->por_el0 = read_sysreg_s(SYS_POR_EL0); + + if (WARN_ON_ONCE(thread->fp_type != FP_STATE_FPSIMD)) + return false; + /* ZA and streaming mode state doesn't move, for now */ + if (thread_sm_enabled(thread) || thread_za_enabled(thread)) + return false; + return true; +} + +/* copy the user register state over, load what the return to user won't */ +int arch_thread_handoff_finish(struct task_struct *src, bool leader) +{ + struct task_struct *dst = current; + struct pt_regs *regs = task_pt_regs(dst); + struct frame_record_meta stackframe = regs->stackframe; + + /* the syscall frame, this is what the return to userspace restores */ + *regs = *task_pt_regs(src); + regs->stackframe = stackframe; + + /* + * Marks our FP state foreign so nothing saves over what's copied in + * below, and frees the SVE/SME buffers, the vector lengths may differ. + */ + fpsimd_flush_thread(); + + preempt_disable(); + + dst->thread.uw = src->thread.uw; + dst->thread.fp_type = FP_STATE_FPSIMD; + memcpy(dst->thread.vl, src->thread.vl, sizeof(dst->thread.vl)); + memcpy(dst->thread.vl_onexec, src->thread.vl_onexec, + sizeof(dst->thread.vl_onexec)); + dst->thread.svcr = src->thread.svcr; + dst->thread.tpidr2_el0 = src->thread.tpidr2_el0; + dst->thread.por_el0 = src->thread.por_el0; + dst->thread.sctlr_user = src->thread.sctlr_user; +#ifdef CONFIG_ARM64_MTE + dst->thread.mte_ctrl = src->thread.mte_ctrl; +#endif +#ifdef CONFIG_ARM64_PTR_AUTH + dst->thread.keys_user = src->thread.keys_user; +#endif + update_tsk_thread_flag(dst, TIF_SVE_VL_INHERIT, + test_tsk_thread_flag(src, TIF_SVE_VL_INHERIT)); + update_tsk_thread_flag(dst, TIF_SME_VL_INHERIT, + test_tsk_thread_flag(src, TIF_SME_VL_INHERIT)); + update_tsk_thread_flag(dst, TIF_TAGGED_ADDR, + test_tsk_thread_flag(src, TIF_TAGGED_ADDR)); + update_tsk_thread_flag(dst, TIF_SSBD, + test_tsk_thread_flag(src, TIF_SSBD)); + if (test_and_clear_tsk_thread_flag(src, TIF_MTE_ASYNC_FAULT)) + set_tsk_thread_flag(dst, TIF_MTE_ASYNC_FAULT); + + /* load what __switch_to() would have, FPSIMD gets restored on exit */ + write_sysreg(dst->thread.uw.tp_value, tpidr_el0); + if (system_supports_tpidr2()) + write_sysreg_s(dst->thread.tpidr2_el0, SYS_TPIDR2_EL0); + if (system_supports_poe()) + write_sysreg_s(dst->thread.por_el0, SYS_POR_EL0); + contextidr_thread_switch(dst); + ptrauth_thread_switch_user(dst); + mte_thread_switch(dst); + update_sctlr_el1(dst->thread.sctlr_user); + + preempt_enable(); + return 0; +} +#endif -- 2.55.0