Re: [Qemu-ppc] PPC hardfloat

[BALATON Zoltan]

On Mon, 26 Aug 2019, Paul Clarke wrote:
> On 8/26/19 5:30 PM, Richard Henderson wrote:
> > On 8/26/19 1:28 PM, BALATON Zoltan wrote:
> > > On Mon, 26 Aug 2019, Richard Henderson wrote:
> > > > That said, qemu-system-ppc64 will *never* use hardfloat, because ppc always
> > > > need the current and correct result of inexact for emulation of the FI bit,
> > > > which requires that we use the softfloat path.
> > >
> > > That's bad news. I hoped that hardfloat for PPC can be implemented and
> > > previously it was thought it could be done after some reorganisation to prevent
> > > always reseting flags by moving them to environment or somewhere else (but I
> > > don't remember the details and maybe I never fully understood it in the first
> > > place). Could you please explain why do you think it's not possible?
> >
> > We only use hardfloat if we do not need to compute whether the current
> > operation produces an inexact exception.  We need not care when the sticky
> > inexact bit is already set.
> >
> > However, for ppc, the FPSCR[FI] bit indicates whether the previous fp operation
> > was inexact.  Thus we need to compute the inexact exception for every single
> > operation.

I still don't get it but I'm obviously missing knowledge about how it 
should work. Is there any good short introduction that I should read about 
it? I've found this:

https://docs.oracle.com/cd/E19957-01/806-3568/ncg_handle.html

but haven't read everything. Why is it not enough to clear host inexact 
bit before FP op then update emulated bit based on what hardware 
signalled? Does the PPC FI bit behave differently than corresponding bit 
on other implementations? Or is reading flags at every FP op too 
expensive? (I'd expect that to be still faster than emulating all FP in 
software.)

> Is there a way tell qemu "I don't care about FPSCR[FI], so use hardfloat" ?
> If not, should there be?

That's also a possibility (likely most guest code ignores it anyway) 
however currently hardfloat is completely disabled for PPC so not sure if 
that's the only reason it's not used yet.

Regards,
BALATON Zoltan