Re: [PATCH] MIPS: Fix MIPS_O32_FP64_SUPPORT for 64bit CPUs before R2

[Jiaxun Yang]

> 2023年5月19日 21:10，Maciej W. Rozycki <macro@xxxxxxxxxxx> 写道：
> 
> On Fri, 19 May 2023, Jiaxun Yang wrote:
> 
>> MIPS_O32_FP64_SUPPORT enables possibility of using all 32 FPRs on 32bit
>> kernel in case CPU implemented FR1. As FR1 is present on all 64bit CPUs
>> following R4000's priviliged spec, there is no reason to limit such support
>> to R2+ CPUs.
> 
> I guess one can do it and still run FPXX software, but I fail to see what 
> gain it provides.  For FP32 it breaks things as accesses to odd-numbered 
> FPRs will no longer get at the high part of a double value and for FP64 
> there are no MTHC1/MFHC1 instructions required to access the high part.

Actually software may access the high part by SDC1/LDC1.
FP32 binaries are still going to run at FR0 mode.

> 
> What problem are you trying to solve?  And how did you verify this patch?

Was trying to deal a proprietary JIT software who want to enable FR1 via prctl
on Loongson-2F with 32 bit kernel.

> 
>> --- a/arch/mips/kernel/fpu-probe.c
>> +++ b/arch/mips/kernel/fpu-probe.c
>> @@ -289,6 +289,23 @@ void cpu_set_fpu_opts(struct cpuinfo_mips *c)
>> c->options |= MIPS_CPU_FRE;
>> }
>> 
>> + /* Fix up FIR for FPU earlier than R2 */
>> + if (!cpu_has_mips_r2_r6) {
>> + c->fpu_id |= MIPS_FPIR_S;
>> +#ifdef CONFIG_CPU_R4K_FPU
>> + /* All known R4000 class FPU implemented double */
>> + c->fpu_id |= MIPS_FPIR_D;
>> +#endif
> 
> Currently all FPUs we support implement double and we require that, so no 
> need to make this piece conditional (I would use IS_ENABLED otherwise, so 
> as not to clutter the source with #ifdef), but `c->fpu_id' is also exposed 
> to the user via ptrace(2), so this has to reflect hardware and not give a 
> synthesized value.

Alas, I thought R2030 class FPU does not have double? Since MIPS-IV spec
says SDC1 is introduced in MIPS II.

Thanks
Jiaxun

> 
>  Maciej