diff mbox

arch/sh: provide unified syscall trap compatible with all SH models

Message ID 20150825030338.GA14922@brightrain.aerifal.cx (mailing list archive)
State Awaiting Upstream
Headers show

Commit Message

Rich Felker Aug. 25, 2015, 3:03 a.m. UTC 
From: Rich Felker <dalias@libc.org>

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>

---

The original proposal for using 0x1f as a unified syscall trap was
reviewed by Jeff Dionne, Yoshinori Sato, and Shumpei Kawasaki for
conflicts with existing trap number assignments, and none were found.
I introduced this proposal as a way to add SH-2 support to musl libc
in a forwards-compatible way, treating SH as a unified architecture,
with the particular intent of supporting the Open Processor
Foundation's J2 Core (SH-2 ISA plus some extensions) and providing a
smooth upgrade path to the future J4 Core (SH-4 equivalent) and other
full-fledged SH platforms with MMU.

There is also some interest in having glibc's SH port support SH-2/J2,
which is more practical with a common syscall ABI; whether this will
actually happen remains uncertain.

Rich



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Comments

Geert Uytterhoeven Aug. 25, 2015, 7:18 a.m. UTC | #1 
Hi Rich,

On Tue, Aug 25, 2015 at 5:03 AM, Rich Felker <dalias@libc.org> wrote:
> From: Rich Felker <dalias@libc.org>
>
> 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>

> --- ../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@linux-m68k.org

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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Rich Felker Aug. 25, 2015, 7:22 a.m. UTC | #2 
On Tue, Aug 25, 2015 at 09:18:44AM +0200, Geert Uytterhoeven wrote:
> Hi Rich,
> 
> On Tue, Aug 25, 2015 at 5:03 AM, Rich Felker <dalias@libc.org> wrote:
> > From: Rich Felker <dalias@libc.org>
> >
> > 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>
> 
> > --- ../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?

It was an unintended side-effect of restructuring the table to be a
list of legacy values. While the actual functional work of this patch
was done months ago, I just noticed that the comments should be
changed yesterday while preparing the patch for submission. I can
rework them to avoid dropping the useful information.

Thanks for the quick initial review!

Rich
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diff mbox

Patch

diff -urp ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2/entry.S ./arch/sh/kernel/cpu/sh2/entry.S
--- ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2/entry.S	2015-08-09 19:54:30.000000000 +0000
+++ ./arch/sh/kernel/cpu/sh2/entry.S	2015-08-24 03:23:34.159387924 +0000
@@ -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
diff -urp ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2a/entry.S ./arch/sh/kernel/cpu/sh2a/entry.S
--- ../linux-4.2-rc6.orig/arch/sh/kernel/cpu/sh2a/entry.S	2015-08-09 19:54:30.000000000 +0000
+++ ./arch/sh/kernel/cpu/sh2a/entry.S	2015-08-24 03:23:58.849386418 +0000
@@ -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
diff -urp ../linux-4.2-rc6.orig/arch/sh/kernel/entry-common.S ./arch/sh/kernel/entry-common.S
--- ../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.
+ * 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"