Re: [PATCH v2 1/2] linux-user: Map low priority rt signals

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

Cc'ing Brian & Taylor

On 12/2/24 21:45, Ilya Leoshkevich wrote:
> Some applications want to use low priority realtime signals (e.g.,
> SIGRTMAX). Currently QEMU cannot map all target realtime signals to
> host signals, and chooses to sacrifice the end of the target realtime
> signal range.
> 
> Change this to the middle of that range, hoping that fewer applications
> will need it.
> 
> Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
> ---
>   linux-user/signal.c | 18 +++++++++++++-----
>   1 file changed, 13 insertions(+), 5 deletions(-)
> 
> diff --git a/linux-user/signal.c b/linux-user/signal.c
> index d3e62ab030f..a81533b563a 100644
> --- a/linux-user/signal.c
> +++ b/linux-user/signal.c
> @@ -511,13 +511,14 @@ static int core_dump_signal(int sig)
>   
>   static void signal_table_init(void)
>   {
> -    int hsig, tsig, count;
> +    int hsig, hsig_count, tsig, tsig_count, tsig_hole, tsig_hole_size, count;
>   
>       /*
> -     * Signals are supported starting from TARGET_SIGRTMIN and going up
> -     * until we run out of host realtime signals.  Glibc uses the lower 2
> -     * RT signals and (hopefully) nobody uses the upper ones.
> -     * This is why SIGRTMIN (34) is generally greater than __SIGRTMIN (32).
> +     * Signals are supported starting from TARGET_SIGRTMIN and up to
> +     * TARGET_SIGRTMAX, potentially with a hole in the middle of this
> +     * range, which, hopefully, nobody uses. Glibc uses the lower 2 RT
> +     * signals; this is why SIGRTMIN (34) is generally greater than
> +     * __SIGRTMIN (32).
>        * To fix this properly we would need to do manual signal delivery
>        * multiplexed over a single host signal.
>        * Attempts for configure "missing" signals via sigaction will be
> @@ -536,9 +537,16 @@ static void signal_table_init(void)
>       host_to_target_signal_table[SIGABRT] = 0;
>       host_to_target_signal_table[hsig++] = TARGET_SIGABRT;
>   
> +    hsig_count = SIGRTMAX - hsig + 1;
> +    tsig_count = TARGET_NSIG - TARGET_SIGRTMIN + 1;
> +    tsig_hole_size = tsig_count - MIN(hsig_count, tsig_count);
> +    tsig_hole = TARGET_SIGRTMIN + (tsig_count - tsig_hole_size) / 2;
>       for (tsig = TARGET_SIGRTMIN;
>            hsig <= SIGRTMAX && tsig <= TARGET_NSIG;
>            hsig++, tsig++) {
> +        if (tsig == tsig_hole) {
> +            tsig += tsig_hole_size;
> +        }
>           host_to_target_signal_table[hsig] = tsig;
>       }
>