Re: [Qemu-devel] [PATCH v2 009/108] linux-user: Set up infrastructure for table-izing syscalls

[Peter Maydell <peter.maydell@linaro.org>]

On 10 June 2018 at 04:00, Richard Henderson
<richard.henderson@linaro.org> wrote:
> At the same time, split out set_robust_list and get_robust_list.
> Put them together, along with their block comment, at the top
> of syscall_table.
>
> Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
> ---
>  linux-user/syscall.c | 87 +++++++++++++++++++++++++++++++++-----------
>  1 file changed, 66 insertions(+), 21 deletions(-)
>
> diff --git a/linux-user/syscall.c b/linux-user/syscall.c
> index 46f123ee13..8678e749ee 100644
> --- a/linux-user/syscall.c
> +++ b/linux-user/syscall.c
> @@ -7947,6 +7947,17 @@ static int host_to_target_cpu_mask(const unsigned long *host_mask,
>      return 0;
>  }
>
> +typedef abi_long impl_fn(void *cpu_env, unsigned num, abi_long arg1,
> +                         abi_long arg2, abi_long arg3, abi_long arg4,
> +                         abi_long arg5, abi_long arg6, abi_long arg7,
> +                         abi_long arg8);
> +
> +#define IMPL(NAME) \
> +static abi_long impl_##NAME(void *cpu_env, unsigned num, abi_long arg1,   \
> +                            abi_long arg2, abi_long arg3, abi_long arg4,  \
> +                            abi_long arg5, abi_long arg6, abi_long arg7,  \
> +                            abi_long arg8)
> +
>  /* This is an internal helper for do_syscall so that it is easier
>   * to have a single return point, so that actions, such as logging
>   * of syscall results, can be performed.
> @@ -11740,23 +11751,6 @@ static abi_long do_syscall1(void *cpu_env, unsigned num, abi_long arg1,
>          return get_errno(safe_tgkill((int)arg1, (int)arg2,
>                           target_to_host_signal(arg3)));
>
> -#ifdef TARGET_NR_set_robust_list
> -    case TARGET_NR_set_robust_list:
> -    case TARGET_NR_get_robust_list:
> -        /* The ABI for supporting robust futexes has userspace pass
> -         * the kernel a pointer to a linked list which is updated by
> -         * userspace after the syscall; the list is walked by the kernel
> -         * when the thread exits. Since the linked list in QEMU guest
> -         * memory isn't a valid linked list for the host and we have
> -         * no way to reliably intercept the thread-death event, we can't
> -         * support these. Silently return ENOSYS so that guest userspace
> -         * falls back to a non-robust futex implementation (which should
> -         * be OK except in the corner case of the guest crashing while
> -         * holding a mutex that is shared with another process via
> -         * shared memory).
> -         */
> -        return -TARGET_ENOSYS;
> -#endif
>
>  #if defined(TARGET_NR_utimensat)
>      case TARGET_NR_utimensat:
> @@ -12412,6 +12406,54 @@ static abi_long do_syscall1(void *cpu_env, unsigned num, abi_long arg1,
>      return ret;
>  }
>
> +/* The default action for a syscall not listed in syscall_table is to
> + * log the missing syscall.  If a syscall is intentionally emulated as
> + * not present, then list it with impl_enosys as the implementation,
> + * which will avoid the logging.
> + */
> +IMPL(enosys)
> +{
> +    return -TARGET_ENOSYS;
> +}
> +
> +/* For a given syscall number, return a function implementing it.
> + * Do this via switch statement instead of table because some targets
> + * do not begin at 0 and others have a large split in the middle of
> + * the numbers.  The compiler should be able to produce a dense table.
> + */
> +static impl_fn *syscall_table(unsigned num)
> +{
> +#define SYSCALL_WITH(X, Y)    case TARGET_NR_##X: return impl_##Y
> +#define SYSCALL(X)            SYSCALL_WITH(X, X)
> +
> +    switch (num) {
> +        /* The ABI for supporting robust futexes has userspace pass
> +         * the kernel a pointer to a linked list which is updated by
> +         * userspace after the syscall; the list is walked by the kernel
> +         * when the thread exits. Since the linked list in QEMU guest
> +         * memory isn't a valid linked list for the host and we have
> +         * no way to reliably intercept the thread-death event, we can't
> +         * support these. Silently return ENOSYS so that guest userspace
> +         * falls back to a non-robust futex implementation (which should
> +         * be OK except in the corner case of the guest crashing while
> +         * holding a mutex that is shared with another process via
> +         * shared memory).
> +         */
> +        SYSCALL_WITH(get_robust_list, enosys);
> +        SYSCALL_WITH(set_robust_list, enosys);
> +
> +        /*
> +         * Other syscalls listed in collation order, with '_' ignored.
> +         */
> +    }

I was expecting this to be a table lookup, something like
   return syscalls[num].impl;

where the other entries in the syscalls[num] structs would be
for instance the strace data we currently have in strace.list.

thanks
-- PMM