Re: [PATCH v3 04/15] target/mips/mips-defs: Introduce CPU_MIPS64 and cpu_type_is_64bit()

[Philippe Mathieu-Daudé]

Hi Richard,

On 1/5/21 9:34 AM, Philippe Mathieu-Daudé wrote:
> On 1/5/21 12:30 AM, Richard Henderson wrote:
>> On 1/4/21 12:11 PM, Philippe Mathieu-Daudé wrote:
>>> MIPS 64-bit ISA is introduced with MIPS3.
>>>
>>> Introduce the CPU_MIPS64 definition aliased to the MIPS3 ISA,
>>> and the cpu_type_is_64bit() method to check if a CPU supports
>>> this ISA (thus is 64-bit).
>>>
>>> Suggested-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
>>> Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
>>> ---
>>>  target/mips/cpu.h       | 5 +++++
>>>  target/mips/mips-defs.h | 4 +++-
>>>  2 files changed, 8 insertions(+), 1 deletion(-)
>>
>> I still don't understand this as an alias, as opposed to a separate bit.
>> (ISA_MIPS3 | CPU_MIPS64R6) does not make sense, because R6 removes 
>> instructions.

Well, FWIW (ISA_MIPS3 | CPU_MIPS64R6) is what we currently have:

#define CPU_MIPS64R6    (CPU_MIPS64R5 | CPU_MIPS32R6 | ISA_MIPS64R6)

#define CPU_MIPS64R5    (CPU_MIPS64R3 | CPU_MIPS32R5 | ISA_MIPS64R5)

#define CPU_MIPS64R3    (CPU_MIPS64R2 | CPU_MIPS32R3 | ISA_MIPS64R3)

#define CPU_MIPS64R2    (CPU_MIPS64 | CPU_MIPS32R2 | ISA_MIPS64R2)

#define CPU_MIPS64      (CPU_MIPS5 | CPU_MIPS32 | ISA_MIPS64)

#define CPU_MIPS5       (CPU_MIPS4 | ISA_MIPS5)

#define CPU_MIPS4       (CPU_MIPS3 | ISA_MIPS4)

#define CPU_MIPS3       (CPU_MIPS2 | ISA_MIPS3)

So CPU_MIPS64R6 -> CPU_MIPS64R5 -> CPU_MIPS64R3 -> CPU_MIPS64R2
 -> CPU_MIPS64 -> CPU_MIPS5 -> CPU_MIPS4 -> CPU_MIPS3 -> ISA_MIPS3

This patch doesn't make it worst :)

> I now understand your concern.
> 
> I based the decodetree logic on the R6 ISA manual, decoding the
> bits from the more recent feature (a leaf) to the root, following
> this tree:
> https://images.anandtech.com/doci/8457/MIPS%20ISA%20Evolution.JPG
> (2.3 Evolution of the Architecture).
> 
> Also "2.1 Historical Perspective":
> 
>   In the MIPS IIITM ISA, 64-bit integers and addresses were added
>   to the instruction set. [...]
>   The MIPS32 Release 6 ISA maintains backward-compatibility, with
>   the exception of a few rarely used instructions, though the use
>   of trap-and-emulate or trap-and-patch; all pre-Release 6 binaries
>   can execute under binary translation. [...]
>   The MIPS64 Architecture is based on the MIPS V ISA and is backward
>   compatible with the MIPS32 Architecture.
> 
>> But if this is an intermediate step,
> 
> As this isn't planned as intermediate step, I'll try to keep
> CPU_MIPS64 as a separate bit and post the result.

I'm not sure it is worth the effort, as I plan to check each ISA /
ASE bit from the CP0_ConfigX bits (similarly target/arm/ does), so
these definitions should disappear eventually.

I.e. for the MSA ASE see:
https://www.mail-archive.com/qemu-devel@nongnu.org/msg767523.html

 /* Check presence of MSA implementation */
 static inline bool ase_msa_available(CPUMIPSState *env)
 {
     return env->CP0_Config3 & (1 << CP0C3_MSAP);
 }

Might I keep your R-b tag for this patch (eventually improving the
commit description with some of the info added in this mail) and
keep going?

Thanks,

Phil.