Re: [PATCH 4/4] MIPS: emulate CPUCFG instruction on older Loongson64 cores

[Jiaxun Yang <jiaxun.yang@flygoat.com>]

On Sat,  2 May 2020 21:38:46 +0800
WANG Xuerui <git@xen0n.name> wrote:

> CPUCFG is the instruction for querying processor characteristics on
> newer Loongson processors, much like CPUID of x86. Since the
> instruction is supposedly designed to provide a unified way to do
> feature detection (without having to, for example, parse
> /proc/cpuinfo which is too heavyweight), it is important to provide
> compatibility for older cores without native support. Fortunately,
> most of the fields can be synthesized without changes to semantics.
> Performance is not really big a concern, because feature detection
> logic is not expected to be invoked very often in typical userland
> applications.
> 
> The instruction can't be emulated on LOONGSON_2EF cores, according to
> FlyGoat's experiments. Because the LWC2 opcode is assigned to other
> valid instructions on 2E and 2F, no RI exception is raised for us to
> intercept. So compatibility is only extended back furthest to
> Loongson-3A1000. Loongson-2K is covered too, as it is basically a
> remix of various blocks from the 3A/3B models from a kernel
> perspective.
> 
> This is lightly based on Loongson's work on their Linux 3.10 fork, for
> being the authority on the right feature flags to fill in, where
> things aren't otherwise discoverable.
> 
> Signed-off-by: WANG Xuerui <git@xen0n.name>
> Cc: Huacai Chen <chenhc@lemote.com>
> Cc: Jiaxun Yang <jiaxun.yang@flygoat.com>
> Cc: Tiezhu Yang <yangtiezhu@loongson.cn>

Reviewed-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Tested-by: Jiaxun Yang <jiaxun.yang@flygoat.com> # Loongson-2K, 3B1500

Thanks for providing me early access of this series so I can perform
reviewing and testing before you posting to the list.

And welcome!

--
Jiaxun Yang

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