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

From: Richard Henderson
Subject: Re: [RFC PATCH 0/3] target/mips: Make the number of TLB entries a CPU property
Date: Tue, 13 Oct 2020 19:22:39 -0700
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

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

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


[*] 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.

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