Re: [PATCH v2] arch/sh: provide unified syscall trap compatible with all SH models

[Rich Felker]

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@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>
> > 
> > ---
> > 
> > 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.
> > --
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