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

Jiaxun Yang
2024-07-12T00:37:07Z

在2024年7月11日七月 下午6:20，Maciej W. Rozycki写道：
> On Thu, 11 Jul 2024, Jiaxun Yang wrote:
>
>> >> that's just one case, what about NaN2008 binaries on a legacy MIPS CPU ?
>> >
>> > It would be good to check with hard-float QEMU configured for writable
>> > FCSR.NAN2008 (which is one way original code was verified) that things
>> > have not regressed. And also what happens if once our emulation has
>> > triggered for the unsupported FCSR.NAN2008 mode, an attempt is made to
>> > flip the mode bit via ptrace(2), e.g. under GDB, which I reckon our
>> > emulation permits for non-legacy CPUs (and which I think should not be
>> > allowed under the new setting).
>>
>> PTrace is working as expected (reflects emulated value).
>
> Yes, sure for reads, but how about *writing* to the bit?

Tested flipping nan2008 bits with ieee754=emulated with ptrace, it works on some extent.
(flipping the bit to unsupported value immediately triggered emulation).

>
>> The actual switchable NaN hardware (M5150, P5600) uses a dedicated Config7
>> bit rather than writable FCSR.NAN2008 to control NaN2008 mode. This is undocumented
>> and not present on some RTL releases. FCSR.NAN2008 is R/O as per The MIPS32 Instruction
>> Set Manual. This renders the purposed test pointless.
>
> Yes, for R6 and arguably R5, but not for R3. Architecture specification
> revisions 3.50 through 5.02 define FCSR.NAN2008 (and also FCSR.ABS2008) as
> either R/O or R/W, at the implementer's discretion, so it is a conforming
> implementation to have these bits writable and our FPU emulator reflects
> it. I won't go into the details here as to why the later revisions of the
> specification have been restricted to the R/O implementation only.
>
> NB architecture specification revisions 3.50 through 5.01 also have the
> FCSR.MAC2008 bit defined, removed altogether later on.

Thanks for the information, I don't have access to those manuals so I was unaware
of that. R/W NAN2008 is prohibited by AVP as well.

I briefly tested NaN2008 distro on QEMU modified with r/w NaN2008 bits in ieee754=
strict mode, it seems working fine.

Thanks
>
> Maciej

--
- Jiaxun