linux-kernel - Re: [PATCH] rust: kernel: Support more jump

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Message-Id: <b3f50745-5351-4e69-b3e2-ede99511e8dc@openatom.club>
Date: Wed, 5 Nov 2025 21:28:39 +0800
From: "Chen Miao" <chenmiao@...natom.club>
To: "Alice Ryhl" <aliceryhl@...gle.com>
Cc: <ojeda@...nel.org>, "Alex Gaynor" <alex.gaynor@...il.com>, 
	"Boqun Feng" <boqun.feng@...il.com>, "Gary Guo" <gary@...yguo.net>, 
	Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	"Benno Lossin" <lossin@...nel.org>, 
	"Andreas Hindborg" <a.hindborg@...nel.org>, 
	"Trevor Gross" <tmgross@...ch.edu>, "Danilo Krummrich" <dakr@...nel.org>, 
	"Steven Rostedt (Google)" <rostedt@...dmis.org>, 
	"open list:RUST" <rust-for-linux@...r.kernel.org>, 
	"open list" <linux-kernel@...r.kernel.org>, 
	<hust-os-kernel-patches@...glegroups.com>
Subject: Re: [PATCH] rust: kernel: Support more jump_label api

On 11/4/2025 10:07 PM, Alice Ryhl wrote:
> On Tue, Nov 04, 2025 at 02:04:17AM +0000, chenmiao wrote:
>> The initial implementation of arch_static_branch was achieved by accessing
>> the offset from the original type. However, this approach extended the
>> path and introduced redundant calculations when dealing with types like
>> `static_key_true/false`, as shown below:
>>
>> ```
>> static_brach_unlikely(tp, tracepoint, key)
>>    => tracepoint->key->key
>>    => &tracepoint->key(static_key_false) == &tracepoint->key.key(static_key)
>>    => off: tracepoint->key - tracepoint
>> ```
>>
>> In practice, the implementation of `arch_static_branch` overlooked many
>> detailed descriptions. To improve clarity, additional comments have been
>> added to the original logic. The approach has been modified to directly
>> locate the corresponding `static_key` instead of using offsets, thereby
>> reducing  computational overhead.
>>
>> If finding the offset from the primitive type is necessary for this
>> implementation, I will abandon this change.
>>
>> Additionally, support for the `static_branch_enable/disable` APIs has been
>> introduced.
>>
>> Signed-off-by: chenmiao <chenmiao@...natom.club>
>> +    ($basety:ty, $branch:expr, $key:path, $keytyp:ty, $field:ident) => {{
>>           let _key: *const $keytyp = ::core::ptr::addr_of!($key);
>> -        let _key: *const $crate::bindings::static_key_false = ::core::ptr::addr_of!((*_key).$field);
>> +        let _key: *const $basety = ::core::ptr::addr_of!((*_key).$field);
>>           let _key: *const $crate::bindings::static_key = _key.cast();
>>   
>>           #[cfg(not(CONFIG_JUMP_LABEL))]
>> @@ -30,7 +71,88 @@ macro_rules! static_branch_unlikely {
>>           }
>>   
>>           #[cfg(CONFIG_JUMP_LABEL)]
>> -        $crate::jump_label::arch_static_branch! { $key, $keytyp, $field, false }
>> +        {
>> +            $crate::jump_label::arch_static_branch! { _key, $branch }
>> +        }
> So ... this is changing from $key to _key. That's replacing the global
> variable with a local variable holding a pointer to the global variable.
> However, the arch_static_branch! macro uses the `sym` operand which
> requires you to pass it the global directly.
>
> Did you try this code? I don't believe it will work.
>
> Alice

I'm very sorry for making a fatal mistake. My misunderstanding of sym led to 
this issue, so I shouldn't make any changes to that part. However, regarding 
the other modifications, I believe it is necessary to support the direct 
passing of structures similar to `static_key_false`, just as in C language.

Chen Miao