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

[Nathan Whitehorn]

On 01/18/16 12:49, John Baldwin wrote:
> 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.
>

Drat. I hadn't appreciated that fpreg was part of the public API when I 
wrote this code! It would be easy enough to follow what mcontext does, 
and that seems like the right solution, but I won't have time for a 
couple of weeks at least. If you do have time, I would be happy to 
review and/or polish patches before that.

Thanks for catching this!
-Nathan