linux-kernel - Re: [PATCH v3 5/5] rust: Add KUNIT tests for bitfield

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Message-ID: <CC11C4AC-A37C-4BB7-820B-7AC257D719D2@nvidia.com>
Date: Sat, 20 Sep 2025 09:33:46 +0000
From: Joel Fernandes <joelagnelf@...dia.com>
To: John Hubbard <jhubbard@...dia.com>
CC: Yury Norov <yury.norov@...il.com>, "linux-kernel@...r.kernel.org"
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Subject: Re: [PATCH v3 5/5] rust: Add KUNIT tests for bitfield

Hello, John,

> On Sep 20, 2025, at 2:52 AM, John Hubbard <jhubbard@...dia.com> wrote:
> 
> On 9/19/25 5:39 PM, Joel Fernandes wrote:
>> On Tue, Sep 16, 2025 at 05:59:18AM -0400, Joel Fernandes wrote:
>> [...]
>>>>> In C also this is valid. If you passed a higher value than what the
>>>>> bitfield can hold, the compiler will still just use the bits that it
>>>>> needs and ignore the rest.
>>>> 
>>>> In C we've got FIELD_{PREP,GET,MODIFY}, implementing the checks.
>>>> So those who want to stay on safe side have a choice.
>>> 
>>> Ah ok. We can add these checks then for the accessors, I will do so in v4.
>> 
>> The C checks use BUILD_BUG_ON, in rust-for-linux we have build_assert but it
>> is fragile and depends on the value being a constant. Since the setter API
>> accepts a run-time value and not a constant, we cannot use this.
>> 
>> Or, we can fail at runtime, but that requires changing the set_* to try_set_*
>> and returning a Result instead of Self. Alternatively, we can have a debug
>> option that panics if the setter API is misued.
> 
> Please no...

True, it requires significant complication.

> 
>> 
>> Thoughts?
>> 
>> Or for the moment, we can keep it simple and filter out / ignore extra bits
>> of the larger value passed (which is what nova-core's register macro bitfield
>> implementation currently does anyway).
>> 
> 
> Yes. Assuming that I'm not completely lost here, you are proposing to
> simply truncate to the size of the bitfield--no panics, no warnings. And
> that's perfectly fine here IMHO.

Ok, thanks. Yeah truncate. This is what register macro also currently does for its bitfields. To the point Jury is making though, the C equivalents do have build checks. We could do that once he have a better build_assert in rust but until then.. hopefully this suffices.

I am currently also research better ways of implementing build_assert.

 - Joel

 

> 
> thanks,
> --
> John Hubbard
>