Re: [PATCH 6/7] ARM: Use TPIDRPRW for current

[Ard Biesheuvel <ardb@kernel.org>]

On Wed, 8 Sept 2021 at 00:00, Keith Packard <keithpac@amazon.com> wrote:
>
> Store current task pointer in CPU thread ID register TPIDRPRW so that
> accessing it doesn't depend on being able to locate thread_info off of
> the kernel stack pointer.
>
> Signed-off-by: Keith Packard <keithpac@amazon.com>
> ---
>  arch/arm/Kconfig                 |  4 +++
>  arch/arm/include/asm/assembler.h |  8 +++++
>  arch/arm/include/asm/current.h   | 52 ++++++++++++++++++++++++++++++++
>  arch/arm/kernel/entry-armv.S     |  4 +++
>  arch/arm/kernel/setup.c          |  1 +
>  arch/arm/kernel/smp.c            |  1 +
>  6 files changed, 70 insertions(+)
>  create mode 100644 arch/arm/include/asm/current.h
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 24804f11302d..414fe23fd5ac 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1172,6 +1172,10 @@ config SMP_ON_UP
>
>           If you don't know what to do here, say Y.
>
> +config CURRENT_POINTER_IN_TPIDRPRW
> +       def_bool y
> +       depends on (CPU_V6K || CPU_V7) && !CPU_V6
> +
>  config ARM_CPU_TOPOLOGY
>         bool "Support cpu topology definition"
>         depends on SMP && CPU_V7
> diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h
> index e2b1fd558bf3..ea12fe3bb589 100644
> --- a/arch/arm/include/asm/assembler.h
> +++ b/arch/arm/include/asm/assembler.h
> @@ -209,6 +209,14 @@
>         mov     \rd, \rd, lsl #THREAD_SIZE_ORDER + PAGE_SHIFT
>         .endm
>
> +/*
> + * Set current task_info
> + * @src: Source register containing task_struct pointer
> + */
> +       .macro  set_current src : req
> +       mcr     p15, 0, \src, c13, c0, 4
> +       .endm
> +
>  /*
>   * Increment/decrement the preempt count.
>   */
> diff --git a/arch/arm/include/asm/current.h b/arch/arm/include/asm/current.h
> new file mode 100644
> index 000000000000..153a2ea18747
> --- /dev/null
> +++ b/arch/arm/include/asm/current.h
> @@ -0,0 +1,52 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +/*
> + * Copyright © 2021 Keith Packard <keithp@keithp.com>
> + */
> +
> +#ifndef _ASM_ARM_CURRENT_H_
> +#define _ASM_ARM_CURRENT_H_
> +
> +#ifndef __ASSEMBLY__
> +
> +register unsigned long current_stack_pointer asm ("sp");
> +
> +/*
> + * Same as asm-generic/current.h, except that we store current
> + * in TPIDRPRW. TPIDRPRW only exists on V6K and V7
> + */
> +#ifdef CONFIG_CURRENT_POINTER_IN_TPIDRPRW
> +
> +struct task_struct;
> +
> +static inline void set_current(struct task_struct *tsk)
> +{
> +       /* Set TPIDRPRW */
> +       asm volatile("mcr p15, 0, %0, c13, c0, 4" : : "r" (tsk) : "memory");
> +}
> +
> +static __always_inline struct task_struct *get_current(void)
> +{
> +       struct task_struct *tsk;
> +
> +       /*
> +        * Read TPIDRPRW.
> +        * We want to allow caching the value, so avoid using volatile and
> +        * instead use a fake stack read to hazard against barrier().
> +        */
> +       asm("mrc p15, 0, %0, c13, c0, 4" : "=r" (tsk)
> +               : "Q" (*(const unsigned long *)current_stack_pointer));
> +
> +       return tsk;
> +}
> +#define current get_current()
> +#else
> +
> +#define set_current(tsk) do {} while (0)
> +
> +#include <asm-generic/current.h>
> +
> +#endif /* CONFIG_SMP */
> +
> +#endif /* __ASSEMBLY__ */
> +
> +#endif /* _ASM_ARM_CURRENT_H_ */
> diff --git a/arch/arm/kernel/entry-armv.S b/arch/arm/kernel/entry-armv.S
> index 0ea8529a4872..db3947ee9c3e 100644
> --- a/arch/arm/kernel/entry-armv.S
> +++ b/arch/arm/kernel/entry-armv.S
> @@ -761,6 +761,10 @@ ENTRY(__switch_to)
>         ldr     r6, [r2, #TI_CPU_DOMAIN]
>  #endif
>         switch_tls r1, r4, r5, r3, r7
> +#ifdef CONFIG_CURRENT_POINTER_IN_TPIDRPRW
> +       ldr     r7, [r2, #TI_TASK]
> +       set_current r7
> +#endif

This is too early: this will cause the thread notification hooks to be
called with current pointing to the new task instead of the old one.

>  #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_SMP)
>         ldr     r7, [r2, #TI_TASK]
>         ldr     r8, =__stack_chk_guard
> diff --git a/arch/arm/kernel/setup.c b/arch/arm/kernel/setup.c
> index d0dc60afe54f..2fdf8c31d6c9 100644
> --- a/arch/arm/kernel/setup.c
> +++ b/arch/arm/kernel/setup.c
> @@ -586,6 +586,7 @@ void __init smp_setup_processor_id(void)
>         u32 mpidr = is_smp() ? read_cpuid_mpidr() & MPIDR_HWID_BITMASK : 0;
>         u32 cpu = MPIDR_AFFINITY_LEVEL(mpidr, 0);
>
> +       set_current(&init_task);
>         cpu_logical_map(0) = cpu;
>         for (i = 1; i < nr_cpu_ids; ++i)
>                 cpu_logical_map(i) = i == cpu ? 0 : i;
> diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
> index 8ccf10b34f08..09771916442a 100644
> --- a/arch/arm/kernel/smp.c
> +++ b/arch/arm/kernel/smp.c
> @@ -410,6 +410,7 @@ asmlinkage void secondary_start_kernel(unsigned int cpu, struct task_struct *tas
>  {
>         struct mm_struct *mm = &init_mm;
>
> +       set_current(task);
>         secondary_biglittle_init();
>
>         /*
> --
> 2.33.0
>