Re: [RFC PATCH 0/3] target/mips: Make the number of TLB entries a CPU property

[Victor Kamensky (kamensky)]

Hi Richard,

Please forgive my cumbersome mailing agent at work.
Please look inline for 'victor>' 

________________________________________
From: Richard Henderson <richard.henderson@linaro.org>
Sent: Tuesday, October 13, 2020 7:22 PM
To: Philippe Mathieu-Daudé; qemu-devel@nongnu.org; Victor Kamensky (kamensky)
Cc: Aleksandar Rikalo; Khem Raj; Aleksandar Markovic; Richard Purdie; Aurelien 
Jarno; Richard Henderson
Subject: Re: [RFC PATCH 0/3] target/mips: Make the number of TLB entries a CPU 
property

On 10/13/20 4:11 PM, Richard Henderson wrote:
> On 10/13/20 6:25 AM, Philippe Mathieu-Daudé wrote:
>> Yocto developers have expressed interest in running MIPS32
>> CPU with custom number of TLB:
>> https://lists.gnu.org/archive/html/qemu-devel/2020-10/msg03428.html
>>
>> Help them by making the number of TLB entries a CPU property,
>> keeping our set of CPU definitions in sync with real hardware.
>
> You mean keeping the 34kf model within qemu in sync, rather than creating a
> nonsense model that doesn't exist.

victor> Question: do current MIPS "generic" qemu cpu models exist for real
victor> out there? I agree my choice was not ideal, but it is not that 
outlandish
victor> and IMO somewhat inline with existence of MIPS generic cpu models.

> Question: is this cpu parameter useful for anything else?

victor> If you are interested here are my testing numbers of how #TLBs impact
victor> user land execution time:
victor> https://lists.openembedded.org/g/openembedded-core/message/143115

> Because the ideal solution for a CI loop is to use one of the mips cpu models
> that has the hw page table walker (CP0C3_PW).  Having qemu being able to 
> refill
> the tlb itself is massively faster.
>
> We do not currently implement a mips cpu that has the PW.  When I downloaded

Bah, "mips32 cpu".

We do have the P5600 that does has it, though the code is wrapped up in
TARGET_MIPS64.  I'll also note that the code could be better placed [*]

> (1) anyone know if the PW incompatible with mips32?

I've since found a copy of the mips32-pra in the wayback machine and have
answered this as "no" -- PW is documented for mips32.

> (2) if not, was there any mips32 hw built with PW
>     that we could model?

But I still don't know about this.

A further question for the Yocto folks: could you make use of a 64-bit kernel
in testing a 32-bit userspace?

victor> Such test does exist and it is part of CI already, it is dubbed as MIPS 
multi-lib
victor> tests where on top mips64 kernel tests are run for n64, n32, and o32 
MIPS ABIs
victor> user-land.
victor> Note mips32 CI in question does cover kernel functionality for example
victor> KLM build and check, SystemTap test, ltp and other kernel operations 
are tested.
victor> I.e it does test 32 bit MIPS kernel as well, but user-land is involved 
in such
victor> tests, and as it was described, it is slow compared to other qemu cases.

Thanks,
Victor

And I guess maybe we should update our recommendations in the docs.  Thoughts
on this, Phil?


r~


[*] Where it is now, it can't be used for gdb (mips_cpu_get_phys_page_debug).
When used there, we should not modify cpu state, i.e. actually insert the PTE
into the MIPS TLB, but we could still make use of the information available.