Re: [PATCH] arch/sh: provide unified syscall trap compatible with all SH models

[Geert Uytterhoeven]

Hi Rich,

On Tue, Aug 25, 2015 at 5:03 AM, Rich Felker <dalias@xxxxxxxx> wrote:
> From: Rich Felker <dalias@xxxxxxxx>
>
> Historically SH-2 Linux (and originally uClinux) used a syscall
> calling convention incompatible with the established SH-3/4 Linux ABI.
> This choice was made because the trap range used by the existing ABI,
> 0x10-0x17, overlaps with the hardware exception/interrupt trap range
> reserved by SH-2, and in particular, with the SH-2A divide-by-zero and
> division-overflow exceptions.
>
> Despite the documented syscall convention using the low bits of the
> trap number to signal the number of arguments the kernel should
> expect, no version of the kernel has ever used this information, nor
> is it useful; all of the registers need to be saved anyway. Therefore,
> it is possible to pick a new trap number, 0x1f, that is both supported
> by all existing SH-3/4 kernels and unassigned as a hardware trap in
> the SH-2 range. This makes it possible to produce SH-2 application
> binaries that are forwards-compatible with running on SH-3/4 kernels
> and to treat SH as a unified platform with varying ISA support levels
> rather than multiple gratuitously-incompatible platforms.
>
> This patch adjusts the range checking SH-2 and SH-2A kernels make for
> the syscall trap to accept the range 0x1f-0x2f rather than just
> 0x20-0x2f. As a result, trap 0x1f now acts as a syscall for all SH
> models.
>
> Signed-off-by: Rich Felker <dalias@xxxxxxxx>

> --- ../linux-4.2-rc6.orig/arch/sh/kernel/entry-common.S 2015-08-09 19:54:30.000000000 +0000
> +++ ./arch/sh/kernel/entry-common.S     2015-08-24 03:33:50.062683702 +0000
> @@ -268,19 +268,20 @@ debug_trap:
>   *     Syscall #: R3
>   *     Arguments #0 to #3: R4--R7
>   *     Arguments #4 to #6: R0, R1, R2
> - *     TRA: (number of arguments + ABI revision) x 4
> + *     TRA>>2: 0x1f, or one of a model-specific set of legacy values:
>   *
> - * This code also handles delegating other traps to the BIOS/gdb stub
> - * according to:
> - *
> - * Trap number
>   * (TRA>>2)    Purpose
>   * --------    -------
>   * 0x00-0x0f   original SH-3/4 syscall ABI (not in general use).
>   * 0x10-0x1f   general SH-3/4 syscall ABI.
> - * 0x20-0x2f   syscall ABI for SH-2 parts.
> - * 0x30-0x3f   debug traps used by the kernel.
> - * 0x40-0xff   Not supported by all parts, so left unhandled.

Why did you remoe the two lines of documentation above?

> + * 0x20-0x2f   original SH-2 syscall ABI.
> + *
> + * SH-2 originally used a separate trap range because several hardware
> + * exceptions fell in the range used for the SH-3/4 syscall ABI. These
> + * are still supported, but trap 0x1f is preferred because it is
> + * compatible with all models.
> + *
> + * This code also handles delegating other traps to the BIOS/gdb stub.
>   *
>   * Note: When we're first called, the TRA value must be shifted
>   * right 2 bits in order to get the value that was used as the "trapa"
Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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