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

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Message-ID: <aQtTRUMUaaUmTAWZ@google.com>
Date: Wed, 5 Nov 2025 13:38:13 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Chen Miao <chenmiao@...natom.club>
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 Wed, Nov 05, 2025 at 09:28:39PM +0800, Chen Miao wrote:
> 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.

It sounds like you are adding a new use-case for this macro. Can you
provide more information for this new feature? It is currently unclear
to me exactly how this will be used.

Alice