Re: [PATCH v9 28/31] common-user: Add safe syscall handling for loongarch64 hosts

[Philippe Mathieu-Daudé]

On 12/14/21 09:01, WANG Xuerui wrote:
> Signed-off-by: WANG Xuerui <git@xen0n.name>
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  .../host/loongarch64/safe-syscall.inc.S       | 81 +++++++++++++++++++
>  1 file changed, 81 insertions(+)
>  create mode 100644 common-user/host/loongarch64/safe-syscall.inc.S
> 
> diff --git a/common-user/host/loongarch64/safe-syscall.inc.S 
> b/common-user/host/loongarch64/safe-syscall.inc.S
> new file mode 100644
> index 0000000000..61dc6554eb
> --- /dev/null
> +++ b/common-user/host/loongarch64/safe-syscall.inc.S
> @@ -0,0 +1,81 @@
> +/*
> + * safe-syscall.inc.S : host-specific assembly fragment
> + * to handle signals occurring at the same time as system calls.
> + * This is intended to be included by common-user/safe-syscall.S
> + *
> + * Ported to LoongArch by WANG Xuerui <git@xen0n.name>
> + *
> + * Based on safe-syscall.inc.S code for every other architecture,
> + * originally written by Richard Henderson <rth@twiddle.net>
> + * Copyright (C) 2018 Linaro, Inc.
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +        .global safe_syscall_base
> +        .global safe_syscall_start
> +        .global safe_syscall_end
> +        .type   safe_syscall_base, @function
> +        .type   safe_syscall_start, @function
> +        .type   safe_syscall_end, @function
> +
> +        /*
> +         * This is the entry point for making a system call. The calling
> +         * convention here is that of a C varargs function with the
> +         * first argument an 'int *' to the signal_pending flag, the
> +         * second one the system call number (as a 'long'), and all further
> +         * arguments being syscall arguments (also 'long').
> +         */
> +safe_syscall_base:
> +        .cfi_startproc
> +        /*
> +         * The syscall calling convention is nearly the same as C:
> +         * we enter with a0 == &signal_pending
> +         *               a1 == syscall number
> +         *               a2 ... a7 == syscall arguments
> +         *               and return the result in a0
> +         * and the syscall instruction needs
> +         *               a7 == syscall number
> +         *               a0 ... a5 == syscall arguments
> +         *               and returns the result in a0
> +         * Shuffle everything around appropriately.
> +         */
> +        move    $t0, $a0        /* signal_pending pointer */
> +        move    $t1, $a1        /* syscall number */
> +        move    $a0, $a2        /* syscall arguments */
> +        move    $a1, $a3
> +        move    $a2, $a4
> +        move    $a3, $a5
> +        move    $a4, $a6
> +        move    $a5, $a7
> +        move    $a7, $t1
> +
> +        /*
> +         * This next sequence of code works in conjunction with the
> +         * rewind_if_safe_syscall_function(). If a signal is taken
> +         * and the interrupted PC is anywhere between 'safe_syscall_start'
> +         * and 'safe_syscall_end' then we rewind it to 'safe_syscall_start'.
> +         * The code sequence must therefore be able to cope with this, and
> +         * the syscall instruction must be the final one in the sequence.
> +         */
> +safe_syscall_start:
> +        /* If signal_pending is non-zero, don't do the call */
> +        ld.w    $t1, $t0, 0
> +        bnez    $t1, 2f
> +        syscall 0
> +safe_syscall_end:
> +        /* code path for having successfully executed the syscall */
> +        li.w    $t2, -4096
> +        bgtu    $a0, $t2, 0f
> +        jr      $ra
> +
> +        /* code path setting errno */
> +0:      sub.d   $a0, $zero, $a0
> +        b       safe_syscall_set_errno_tail
> +
> +        /* code path when we didn't execute the syscall */
> +2:      li.w    $a0, QEMU_ERESTARTSYS
> +        b       safe_syscall_set_errno_tail
> +        .cfi_endproc
> +        .size   safe_syscall_base, .-safe_syscall_base
> 

Why not rename 0 -> set_errno and 2 -> syscall_not_executed
for readability? (and eventually drop the comments).

Otherwise:
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>