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

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Message-ID: <aQtbeWpRD7ip8-2A@google.com>
Date: Wed, 5 Nov 2025 14:13:13 +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 09:55:27PM +0800, Chen Miao wrote:
> On 11/5/2025 9:38 PM, Alice Ryhl wrote:
> > 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
> 
> If there's a binding-required driver implementation in the future where a key 
> function uses if (static_branch_unlikely(&zoned_enabled))— defined by 
> DEFINE_STATIC_KEY_FALSE(zoned_enabled);— then in Rust we can directly 
> implement it using if static_branch_unlikely!(zoned_enabled), without having 
> to call it via if static_branch_unlikely!(zoned_enabled, 
> bindings::static_key_false, key).
> 
> static_branch_unlikely!(zoned_enabled) instead of 
> static_branch_unlikely!(zoned_enabled, bindings::static_key_false, key).

In general, you would never use "static_key_false" as the second
argument to static_branch_unlikely!. The second argument is the name of
the struct *containing* a field of type static_key_false.

I guess your point is that there's no way to use the macro right now if
the global is a bare static_key_false that is not wrapped in a struct?

Alice