Re: [PATCH 13/24] bsd-user/arm/target_arch_thread.h: Routines to create and switch to a thread

[Kyle Evans]

On Tue, Oct 19, 2021 at 11:45 AM Warner Losh <imp@bsdimp.com> wrote:
>
> Implement target_thread_init (to create a thread) and target_set_upcall
> (to switch to a thread) for arm.
>
> Signed-off-by: Stacey Son <sson@FreeBSD.org>
> Signed-off-by: Klye Evans <kevans@FreeBSD.org>
> Signed-off-by: Warner Losh <imp@bsdimp.com>
> ---
>  bsd-user/arm/target_arch_thread.h | 71 +++++++++++++++++++++++++++++++
>  1 file changed, 71 insertions(+)
>  create mode 100644 bsd-user/arm/target_arch_thread.h
>
> diff --git a/bsd-user/arm/target_arch_thread.h 
> b/bsd-user/arm/target_arch_thread.h
> new file mode 100644
> index 0000000000..317364bb84
> --- /dev/null
> +++ b/bsd-user/arm/target_arch_thread.h
> @@ -0,0 +1,71 @@
> +/*
> + *  arm thread support
> + *
> + *  Copyright (c) 2013 Stacey D. Son
> + *
> + *  This program is free software; you can redistribute it and/or modify
> + *  it under the terms of the GNU General Public License as published by
> + *  the Free Software Foundation; either version 2 of the License, or
> + *  (at your option) any later version.
> + *
> + *  This program is distributed in the hope that it will be useful,
> + *  but WITHOUT ANY WARRANTY; without even the implied warranty of
> + *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
> + *  GNU General Public License for more details.
> + *
> + *  You should have received a copy of the GNU General Public License
> + *  along with this program; if not, see <http://www.gnu.org/licenses/>.
> + */
> +#ifndef _TARGET_ARCH_THREAD_H_
> +#define _TARGET_ARCH_THREAD_H_
> +
> +/* Compare to arm/arm/vm_machdep.c cpu_set_upcall_kse() */
> +static inline void target_thread_set_upcall(CPUARMState *regs, abi_ulong 
> entry,
> +    abi_ulong arg, abi_ulong stack_base, abi_ulong stack_size)
> +{
> +    abi_ulong sp;
> +
> +    /*
> +     * Make sure the stack is properly aligned.
> +     * arm/include/param.h (STACKLIGN() macro)
> +     */
> +    sp = (u_int)((stack_base + stack_size) -
> +        sizeof(struct target_trapframe)) & ~0x7;
> +
> +    /* sp = stack base */
> +    regs->regs[13] = sp;
> +    /* pc = start function entry */
> +    regs->regs[15] = entry & 0xfffffffe;
> +    /* r0 = arg */
> +    regs->regs[0] = arg;
> +    regs->spsr = ARM_CPU_MODE_USR;
> +    if (entry & 0x1) {
> +        regs->spsr |= CPSR_T;
> +    }
> +}
> +
> +static inline void target_thread_init(struct target_pt_regs *regs,
> +        struct image_info *infop)
> +{
> +    abi_long stack = infop->start_stack;
> +    memset(regs, 0, sizeof(*regs));
> +    regs->ARM_cpsr = 0x10;
> +    if (infop->entry & 1) {
> +        regs->ARM_cpsr |= CPSR_T;
> +    }
> +    regs->ARM_pc = infop->entry & 0xfffffffe;
> +    regs->ARM_sp = infop->start_stack;
> +    if (bsd_type == target_freebsd) {
> +        regs->ARM_lr = infop->entry & 0xfffffffe;
> +    }
> +    /* FIXME - what to for failure of get_user()? */
> +    get_user_ual(regs->ARM_r2, stack + 8); /* envp */
> +    get_user_ual(regs->ARM_r1, stack + 4); /* envp */
> +    /* XXX: it seems that r0 is zeroed after ! */
> +    regs->ARM_r0 = 0;
> +    /* For uClinux PIC binaries.  */
> +    /* XXX: Linux does this only on ARM with no MMU (do we care ?) */
> +    regs->ARM_r10 = infop->start_data;
> +}
> +
> +#endif /* !_TARGET_ARCH_THREAD_H_ */
> --
> 2.32.0
>

I think it's obvious enough to folks already familiar with ARM, but I
wonder if we shouldn't add in some basic commentary about the thumb
bits above. Something like:

/*
 * The low bit in an entry point indicates a thumb instruction; the entry point
 * can't actually exist at this address because it must be 16- or 32-
bit aligned.
 * The low bit gets masked off and the T bit in CSPR is twiddled to
indicate thumb.
 */