Re: [PATCH resend] MIPS: Allow FPU emulator to use non-stack area.

[David Daney]

On 10/06/2014 05:29 PM, Andy Lutomirski wrote:
> On Mon, Oct 6, 2014 at 5:28 PM, Andrew Pinski <pinskia@xxxxxxxxx> wrote:
> > On Mon, Oct 6, 2014 at 5:21 PM, Rich Felker <dalias@xxxxxxxx> wrote:
> > > On Mon, Oct 06, 2014 at 05:11:38PM -0700, Andrew Pinski wrote:
> > > > On Mon, Oct 6, 2014 at 5:05 PM, Rich Felker <dalias@xxxxxxxx> wrote:
> > > > > On Mon, Oct 06, 2014 at 04:48:52PM -0700, David Daney wrote:
> > > > > > On 10/06/2014 04:38 PM, Andy Lutomirski wrote:
> > > > > > > On 10/06/2014 02:58 PM, Rich Felker wrote:
> > > > > > > > On Mon, Oct 06, 2014 at 02:45:29PM -0700, David Daney wrote:
> > > > > > [...]
> > > > > > > > This is a huge ill-designed mess.
> > > > > > >
> > > > > > > Amen.
> > > > > > >
> > > > > > > Can the kernel not just emulate the instructions directly?
> > > > > >
> > > > > > In theory it could, but since there can be implementation defined
> > > > > > instructions, there is no way to achieve full instruction set
> > > > > > coverage for all possible machines.
> > > > >
> > > > > Is the issue really implementation-defined instructions with delay
> > > > > slots? If so it sounds like a made-up issue. They're not going to
> > > > > occur in real binaries. Certainly a compiler is not going to generate
> > > > > implementation-defined instructions, and if you're writing the asm by
> > > > > hand, you just don't put floating point instructions in the delay
> > > > > slot.
> > > >
> > > > It is not the instruction with delay slot but rather the instruction
> > > > in the delay slot itself.
> > >
> > > An instruction in the delay slot for the instruction being emulated?
> > > How would that arise? Are there floating point instructions with delay
> > > slots?
> >
> > Yes branches.
>
> I admit I have no idea what's going here, but I find it hard to
> believe that having the kernel fix this up for new code is desirable.
> Unless MIPS can round-trip a trap *very* quickly, performance will be
> awful for any code that has this problem.

It is FPU *emulation*, of course the performance will suck.  We don't 
care about performance, we just want it to execute correctly.

David Daney

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