Re: [PATCH] [SPARC] callfuncs.exp: avoid spurious register differences in sparc64 targets.

[jose.marchesi@oracle.com (Jose E. Marchesi)]

    > +	-re  "^pstate\[ \t\]+\[^\r\n\]+\r\n" {
    > +	    if [istarget "sparc64-*-linux-gnu"] {
    > +		# Filter out the pstate register, since in sparc64
    > +		# targets the Linux kernel disables pstate.PEF when
    > +		# returning from traps, giving spurious differences.
    
    Isn't this a kernel bug?  It sounds like it's impossible to debug FPU
    code if you e.g. step over FPU instructions?

No, it is not a kernel bug.  It is a consequence of how the sparc
kernel port handles the restoring of FP registers clobbered by kernel
code.  As far as I understand it:

When an user program uses the FPU in any way (any instruction
referencing FP registers for example) a fp_disabled trap is triggered
and the kernel enables the FPU so the user program can happily continue
executing FPU instructions.

If at some point the user program traps into the kernel (syscall, or
whatever) with the FPU activated the kernel saves whatever FP registers
it may clobber in the corresponding thread struct.  Then it disables the
FPU and returns to the user program.

Then, if the user program uses the FPU again, another fp_disabled trap
is triggered, and the kernel will both re-activate the FPU and restore
all the "dirty" FP registers that were clobbered in the previous trap.