svn commit: r279189 - in head/sys/powerpc: aim fpu include powerpc

[John Baldwin]

On Sunday, February 22, 2015 09:40:28 PM Nathan Whitehorn wrote:
> Author: nwhitehorn
> Date: Sun Feb 22 21:40:27 2015
> New Revision: 279189
> URL: https://svnweb.freebsd.org/changeset/base/279189
> 
> Log:
>   Kernel support for the Vector-Scalar eXtension (VSX) found on the POWER7
>   and POWER8. This instruction set unifies the 32 64-bit scalar floating
>   point registers with the 32 128-bit vector registers into a single bank
>   of 64 128-bit registers. Kernel support mostly amounts to saving and
>   restoring the wider version of the floating point registers and making
>   sure that both scalar FP and vector registers are enabled once a VSX
>   instruction is executed. get_mcontext() and friends currently cannot
>   see the high bits, which will require a little more work.
>   
>   As the system compiler (GCC 4.2) does not support VSX, making use of this
>   from userland requires either newer GCC or clang.
>   
>   Relnotes:	yes
>   Sponsored by:	FreeBSD Foundation
> 
> Modified:
>   head/sys/powerpc/aim/trap.c
>   head/sys/powerpc/aim/trap_subr64.S
>   head/sys/powerpc/fpu/fpu_emu.c
>   head/sys/powerpc/fpu/fpu_explode.c
>   head/sys/powerpc/include/cpu.h
>   head/sys/powerpc/include/pcb.h
>   head/sys/powerpc/include/psl.h
>   head/sys/powerpc/include/reg.h
>   head/sys/powerpc/include/trap.h
>   head/sys/powerpc/powerpc/cpu.c
>   head/sys/powerpc/powerpc/db_trace.c
>   head/sys/powerpc/powerpc/exec_machdep.c
>   head/sys/powerpc/powerpc/fpu.c
> 
> Modified: head/sys/powerpc/include/reg.h
> ==============================================================================
> --- head/sys/powerpc/include/reg.h	Sun Feb 22 21:32:57 2015	(r279188)
> +++ head/sys/powerpc/include/reg.h	Sun Feb 22 21:40:27 2015	(r279189)
> @@ -20,7 +20,10 @@ struct reg {
>  
>  /* Must match pcb.pcb_fpu */
>  struct fpreg {
> -	double fpreg[32];
> +	union {
> +		double fpr;
> +		uint64_t vsr[2];
> +	} fpreg[32];
>  	double fpscr;
>  };

This breaks the ABI of struct fpreg which changes the format of coredumps.
It also breaks the ABI of older versions of programs (such as debuggers) that
use ptrace(PT_GETFPREGS) (the kernel will now overflow the user-allocated
buffer if a debugger built on 10.x is run under an 11.0 kernel, and
PT_SETFPREGS is going to also buffer overflow and store random garbage in the
FP regs).  Did you mean to alter the structure's layout?

I can maybe fix upstream gdb to cope with either size, but it's a bit of a
PITA.  In particular, gdb assumes that all the floating point registers in
struct fpreg are in a packed array (so it can use starting_offset + 8 * n to
extract register 'n' and only the initial offset is something that different
platform targets have to configure), so fixing this means having to add a lot
of special cases and duplicate code that is otherwise shared across
platforms.

Hmm, I see that you preserved the ABI of mcontext by just copying the
doubles.  I think you should do the same for 'struct fpreg' restoring its
ABI and we can use MD ptrace requests to fetch the VSX state.  This is what
Linux effectively does.

-- 
John Baldwin