Re: [PATCH v3] MIPS: Implement ieee754 NAN2008 emulation mode

[YunQiang Su]

Maciej W. Rozycki <macro@xxxxxxxxxxx> 于2024年7月15日周一 22:02写道：
>
> On Mon, 15 Jul 2024, Jiaxun Yang wrote:
>
> > >  I don't know what prctl(2) has to do with this.  If you don't implement
> > > this part, then your change will cause Linux to behave inconsistently and
> > > therefore I'll have to NAK it.
> >
> > I think your concern was regarding user space application needs to set NaN2008 bits
> > at runtime?
>
>  Nope, following the objective of your change: the EF_MIPS_NAN2008 ELF
> file header flag instructs the kernel to choose between hardware and
> emulated hard float and that's not supposed to change later on throughout
> the life of the program, because it's not something the program can do
> itself, because writes to FCSR.NAN2008 are ignored by hardware.  And it's
> not a functional regression, because flipping FCSR.NAN2008 isn't allowed
> by hardware concerned anyway, we just want to have it consistent including
> the debugger interface.
>
> > >  It's not much to do anyway, as I have prepared `ptrace_setfcr31' already
> > > to handle masking correctly, so all you have to do is to set the mask as
> > > required for the right thing to happen.  I shouldn't have needed to point
> > > you at it though, as that code is easy to find.
> >
> > I think I got your point, will try to implement it.
>
>  Thank you.
>
> > >  This doesn't matter either, as your change only addresses the case where
> > > FCSR.NAN2008 isn't writable anyway, which is the sole reason you want to
> > > switch between native hard float support and emulation, doesn't it?
> > >
> > >  In fact where FCSR.NAN2008 is writable your new mode has to be equivalent
> > > to "ieee754=strict", because then there is no need to trigger emulation
> > > for either NaN mode.  Please do verify that this is the case.
> >
> > This had been verified with perf math-emu counters to ensure no unnecessary emulation
> > is triggered.
>
>  Thanks.
>
> > >  That doesn't matter for us here (and I have a bad suspicion anyway), but
> > > the Debian team is of course free to do what they want here, the GNU GPL
> > > applies.
> >
> > We care about our downstream users, don't we?
>
>  There is a balance for us to keep.  Requests made have to be reasonable
> and code contributed has to be architected well and meet quality criteria.
> Every change carries its associated cost and especially with the limited
> manpower available we can't afford having a technical debt created.  Any
> unclean piece of code accepted will strike us back sooner or later.
>
> > They asked me to help and that was my solution. I sincerely want to get this change upstreamed
> > to cover some downstream use cases.
>
>  If it's your own genuine from-scratch implementation, then I have more
> faith in it.
>
> > I don't know what theory do you have here, but that's all stories behind.
>
>  I have seen odd requests and code changes stemming from embarassing lack
> of understanding how things work with the MIPS architecture.
>

I won't think that your reply makes any sense since you have a typo here.  ;)

There is nothing in the world perfect I guess, while we need to
continue our life
and there are always new problems needing to be solved in our life.

Once the problem happens, we need a solution for it.
I think that a solution is a good one if it
    won't break something more
    we cannot forecast it will break something more in future
    It won't be very hard to rollback or drop it in future

I don't think that it is a good behavioral pattern to say "let's
destroy the world
since it is not perfect".  Anyway we live in a world full of tradeoffs.

> > >  And also they can always use the "nofpu" kernel parameter to run their
> > > verification.  I used it for mine back at ImgTec before 2008 NaN hardware
> > > was available, also to verify emulation, which I wrote too.  Perhaps that
> > > is also the right solution for Debian actually?
> >
> > I'll suggest to them, thanks.
>
>  I note that it's been like this since 2015 and it has been documented:
>
>         ieee754=        [MIPS] Select IEEE Std 754 conformance mode
>                         Format: { strict | legacy | 2008 | relaxed }
>                         Default: strict
> [...]
>                         The FPU emulator is always able to support both NaN
>                         encodings, so if no FPU hardware is present or it has
>                         been disabled with 'nofpu', then the settings of
>                         'legacy' and '2008' strap the emulator accordingly,
>                         'relaxed' straps the emulator for both legacy-NaN and
>                         2008-NaN, whereas 'strict' enables legacy-NaN only on
>                         legacy processors and both NaN encodings on MIPS32 or
>                         MIPS64 CPUs.
>
> (see the part following the last comma in particular).  It usually makes
> sense to read documentation and I'd expect MIPS Debian port experts to do
> it from time to time.
>
>   Maciej
>