| Message ID | 20150825212311.GM32742@brightrain.aerifal.cx (mailing list archive) |
|---|---|
| State | Awaiting Upstream |
| Headers | show |
On 08/25/2015 04:23 PM, Rich Felker 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. Except that sh2 is a nommu platform, requiring different binary formats. You can't run a standard ELF binary on sh2, and sh3/4 usually don't have fdpic or elf loaders configured in. Even if you could, basic system calls like fork() don't work on a nommu system. The workarounds for fork() not working (re-exec yourself signaling that this is the second instance, then have main() detect the signal and call a different entry point) are intrusive and cumbersome enough that you generally don't want them in programs that _don't_ need them. Therefore you historically always distinguish at build time whether to include nommu codepaths or not. This is why uClibc doesn't include those in nommu builds of the library, so you can easily comple-time probe for it even when cross compiling. (Speaking of which, I would very much like to be able to pass a NULL exec to signal "re-exec this same program" without relying on /proc to be mounted for /proc/self/exe, but alas the kernel can't do that.) I agree it doesn't hurt anything (other than new binaries built to use it not running on old kernels, but that's pretty much the case for fdpic anyway), but what is the actual _advantage_ of this patch? Rob -- To unsubscribe from this list: send the line "unsubscribe linux-sh" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Ping? On Tue, Aug 25, 2015 at 05:23:11PM -0400, Rich Felker 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> > > --- > > 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. > > Version 2 of the patch addresses issues raised by Geert Uytterhoeven > with the changes to comments in entry-common.S. > > Rich > > > > 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 2015-08-25 21:08:15.835463227 +0000 > @@ -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. > -- > To unsubscribe from this list: send the line "unsubscribe linux-sh" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-sh" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Mon, Sep 14, 2015 at 10:17:50PM -0500, Rob Landley wrote: > On 08/25/2015 04:23 PM, Rich Felker 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. > > Except that sh2 is a nommu platform, requiring different binary formats. > You can't run a standard ELF binary on sh2, You can. It's a near-trivial patch to make it work now, though I want to polish it more before sending it upstream. > and sh3/4 usually don't have > fdpic or elf loaders configured in. The normal ELF loader loads FDPIC just fine; it just doesn't float the data segment separately. At first it didn't work for me due to limitations in my entry-point code, but now even qemu-sh4eb runs my FDPIC binaries fine. Of course it would be really _nice_ to have separately floating text/data even on systems with MMU. In theory the code in binfmt_elf_fdpic.c supports this, but it's just not enabled in kconfig. > Even if you could, basic system calls like fork() don't work on a nommu > system. Obviously programs that need fork will not work. But the vast majority of software does not need fork -- or if it does need it, is using it gratuitously, and should be using posix_spawn which would give you both major performance boosts and NOMMU support. And of course the other direction, which is the interesting one, of course works perfectly fine -- software that does not expect fork to be available because it was built for NOMMU certainly does not care if fork is available on the system it actually runs on. > I agree it doesn't hurt anything (other than new binaries built to use > it not running on old kernels, but that's pretty much the case for fdpic > anyway), but what is the actual _advantage_ of this patch? I've been over this many times, but I can summarize again. The big points are: - Ability to run & debug the exact same binaries you're going to deploy to NOMMU hardware on a much nicer system with memory protection, dev tools, etc. (or qemu-sh4eb). - Establishing SH as a mature, serious platform with ISA levels and only minimal hard ABI splits (like endianness, hard/soft float, etc.) rather than having each ISA level be its own completely separate and incompatible platform. Do these seem unreasonable? Rich -- To unsubscribe from this list: send the line "unsubscribe linux-sh" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Ping. On Tue, Sep 15, 2015 at 12:11:24AM -0400, Rich Felker wrote: > Ping? > > On Tue, Aug 25, 2015 at 05:23:11PM -0400, Rich Felker 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> > > > > --- > > > > 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. > > > > Version 2 of the patch addresses issues raised by Geert Uytterhoeven > > with the changes to comments in entry-common.S. > > > > Rich > > > > > > > > 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 2015-08-25 21:08:15.835463227 +0000 > > @@ -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. > > -- > > To unsubscribe from this list: send the line "unsubscribe linux-sh" in > > the body of a message to majordomo@vger.kernel.org > > More majordomo info at http://vger.kernel.org/majordomo-info.html > -- > To unsubscribe from this list: send the line "unsubscribe linux-sh" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line "unsubscribe linux-sh" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
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 2015-08-25 21:08:15.835463227 +0000 @@ -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.