[PATCH] arm64: extract a field correctly in cpuid_feature_extract_field()

Suzuki K. Poulose Suzuki.Poulose at arm.com
Tue Nov 17 01:27:19 PST 2015

On 17/11/15 07:15, Ard Biesheuvel wrote:
> On 17 November 2015 at 06:05, AKASHI Takahiro
> <takahiro.akashi at linaro.org> wrote:
>> Basically, cpuid_feature_extract_field() does shift-left and then
>> shift-right to extract a specific field in an operand. But
>> a shift-left'ed value is casted to 's64' and so a succeeding shift-right
>> operation results in creating a sign-extended (and bogus) value.
> This is intentional. This function was created specifically for
> extracting CPU feature fields, which are signed 4-bit quantities,
> where positive values represent incremental functionality, and
> negative values are reserved. This is poorly documented in the ARM ARM
> though.

Right. Akash's fix could break other pieces (like FP/ASIMD support in IDAA64PFR0
where, 0xf => function not implemented).

> Using this function for extracting 4-bit unsigned values is a mistake.

I will take a look at this one.

Thanks for pointing it out.


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