Re: [PATCH 1/2] common-user/host/ppc: Implement safe-syscall.inc.S

[Daniel Henrique Barboza <danielhb413@gmail.com>]

On 7/29/22 14:21, Richard Henderson wrote:
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---

Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>

>   common-user/host/ppc/safe-syscall.inc.S | 107 ++++++++++++++++++++++++
>   1 file changed, 107 insertions(+)
>   create mode 100644 common-user/host/ppc/safe-syscall.inc.S
> 
> diff --git a/common-user/host/ppc/safe-syscall.inc.S b/common-user/host/ppc/safe-syscall.inc.S
> new file mode 100644
> index 0000000000..0851f6c0b8
> --- /dev/null
> +++ b/common-user/host/ppc/safe-syscall.inc.S
> @@ -0,0 +1,107 @@
> +/*
> + * 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
> + *
> + * Copyright (C) 2022 Linaro, Ltd.
> + *
> + * This work is licensed under the terms of the GNU GPL, version 2 or later.
> + * See the COPYING file in the top-level directory.
> + */
> +
> +/*
> + * Standardize on the _CALL_FOO symbols used by GCC:
> + * Apple XCode does not define _CALL_DARWIN.
> + * Clang defines _CALL_ELF (64-bit) but not _CALL_SYSV (32-bit).
> + */
> +#if !defined(_CALL_SYSV) && \
> +    !defined(_CALL_DARWIN) && \
> +    !defined(_CALL_AIX) && \
> +    !defined(_CALL_ELF)
> +# if defined(__APPLE__)
> +#  define _CALL_DARWIN
> +# elif defined(__ELF__) && TCG_TARGET_REG_BITS == 32
> +#  define _CALL_SYSV
> +# else
> +#  error "Unknown ABI"
> +# endif
> +#endif
> +
> +#ifndef _CALL_SYSV
> +# error "Unsupported ABI"
> +#endif
> +
> +
> +        .global safe_syscall_base
> +        .global safe_syscall_start
> +        .global safe_syscall_end
> +        .type   safe_syscall_base, @function
> +
> +        .text
> +
> +        /*
> +         * 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
> +        stwu    1, -8(1)
> +        .cfi_def_cfa_offset 8
> +        stw     30, 4(1)
> +        .cfi_offset 30, -4
> +
> +        /*
> +         * We enter with r3 == &signal_pending
> +         *               r4 == syscall number
> +         *               r5 ... r10 == syscall arguments
> +         *               and return the result in r3
> +         * and the syscall instruction needs
> +         *               r0 == syscall number
> +         *               r3 ... r8 == syscall arguments
> +         *               and returns the result in r3
> +         * Shuffle everything around appropriately.
> +         */
> +        mr      30, 3           /* signal_pending */
> +        mr      0, 4            /* syscall number */
> +        mr      3, 5            /* syscall arguments */
> +        mr      4, 6
> +        mr      5, 7
> +        mr      6, 8
> +        mr      7, 9
> +        mr      8, 10
> +
> +        /*
> +         * 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 */
> +        lwz     12, 0(30)
> +        cmpwi   0, 12, 0
> +        bne-    2f
> +        sc
> +safe_syscall_end:
> +        /* code path when we did execute the syscall */
> +        lwz     30, 4(1)        /* restore r30 */
> +        addi    1, 1, 8         /* restore stack */
> +        .cfi_restore 30
> +        .cfi_def_cfa_offset 0
> +        bnslr+                  /* return on success */
> +        b       safe_syscall_set_errno_tail
> +
> +        /* code path when we didn't execute the syscall */
> +2:      lwz     30, 4(1)
> +        addi    1, 1, 8
> +        addi    3, 0, QEMU_ERESTARTSYS
> +        b       safe_syscall_set_errno_tail
> +
> +        .cfi_endproc
> +
> +        .size   safe_syscall_base, .-safe_syscall_base