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sh: provide unified syscall trap compatible with all SH models

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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@libc.org>
This commit is contained in:
Rich Felker 2015-12-17 01:04:25 +00:00
parent 940d4113f3
commit 3623d13821
3 changed files with 23 additions and 14 deletions
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8
arch/sh/kernel/cpu/sh2/entry.S
View File

@ -144,9 +144,9 @@ ENTRY(exception_handler)
mov #64,r8
cmp/hs r8,r9
bt interrupt_entry ! vec >= 64 is interrupt
mov #32,r8
mov #31,r8
cmp/hs r8,r9
bt trap_entry ! 64 > vec >= 32 is trap
bt trap_entry ! 64 > vec >= 31 is trap
mov.l 4f,r8
mov r9,r4
@ -178,9 +178,9 @@ interrupt_entry:
trap_entry:
mov #0x30,r8
cmp/ge r8,r9 ! vector 0x20-0x2f is systemcall
cmp/ge r8,r9 ! vector 0x1f-0x2f is systemcall
bt 1f
add #-0x10,r9 ! convert SH2 to SH3/4 ABI
mov #0x1f,r9 ! convert to unified SH2/3/4 trap number
1:
shll2 r9 ! TRA
bra system_call ! jump common systemcall entry

8
arch/sh/kernel/cpu/sh2a/entry.S
View File

@ -109,9 +109,9 @@ ENTRY(exception_handler)
mov #64,r8
cmp/hs r8,r9
bt interrupt_entry ! vec >= 64 is interrupt
mov #32,r8
mov #31,r8
cmp/hs r8,r9
bt trap_entry ! 64 > vec >= 32 is trap
bt trap_entry ! 64 > vec >= 31 is trap
mov.l 4f,r8
mov r9,r4
@ -143,9 +143,9 @@ interrupt_entry:
trap_entry:
mov #0x30,r8
cmp/ge r8,r9 ! vector 0x20-0x2f is systemcall
cmp/ge r8,r9 ! vector 0x1f-0x2f is systemcall
bt 1f
add #-0x10,r9 ! convert SH2 to SH3/4 ABI
mov #0x1f,r9 ! convert to unified SH2/3/4 trap number
1:
shll2 r9 ! TRA
bra system_call ! jump common systemcall entry

21
arch/sh/kernel/entry-common.S
View File

@ -268,20 +268,29 @@ 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: See following table.
*
* 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.
* 0x1f unified SH-2/3/4 syscall ABI (preferred).
* 0x20-0x2f original SH-2 syscall ABI.
* 0x30-0x3f debug traps used by the kernel.
* 0x40-0xff Not supported by all parts, so left unhandled.
*
* For making system calls, any trap number in the range for the
* given cpu model may be used, but the unified trap number 0x1f is
* preferred for compatibility with all models.
*
* The low bits of the trap number were once documented as matching
* the number of arguments, but they were never actually used as such
* by the kernel. SH-2 originally used its own separate trap range
* because several hardware exceptions fell in the range used for the
* SH-3/4 syscall ABI.
*
* 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"
* argument.