Re: [PATCH v4 01/11] xen: Implement xen/alternative-call.h for use in common code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.09.2021 21:06, Daniel P. Smith wrote:
> --- /dev/null
> +++ b/xen/include/xen/alternative-call.h
> @@ -0,0 +1,63 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef XEN_ALTERNATIVE_CALL
> +#define XEN_ALTERNATIVE_CALL
> +
> +/*
> + * Some subsystems in Xen may have multiple implementions, which can be
> + * resolved to a single implementation at boot time.  By default, this will
> + * result in the use of function pointers.
> + *
> + * Some architectures may have mechanisms for dynamically modifying .text.
> + * Using this mechnaism, function pointers can be converted to direct calls
> + * which are typically more efficient at runtime.
> + *
> + * For architectures to support:
> + *
> + * - Implement alternative_{,v}call() in asm/alternative.h.  Code generation
> + *   requirements are to emit a function pointer call at build time, and 
> stash
> + *   enough metadata to simplify the call at boot once the implementation has
> + *   been resolved.
> + * - Select ALTERNATIVE_CALL in Kconfig.
> + *
> + * To use:
> + *
> + * Consider the following simplified example.
> + *
> + *  1) struct foo_ops __alt_call_maybe_initdata ops;
> + *
> + *  2) const struct foo_ops __initconst foo_a_ops = { ... };
> + *     const struct foo_ops __initconst foo_b_ops = { ... };
> + *
> + *     void foo_init(void)
> + *     {
> + *         ...
> + *         if ( use_impl_a )
> + *             ops = *foo_a_ops;
> + *         else if ( use_impl_b )
> + *             ops = *foo_b_ops;
> + *         ...
> + *     }
> + *
> + *  3) alternative_call(ops.bar, ...);
> + *
> + * There needs to a single ops object (1) which will eventually contain the
> + * function pointers.  This should be populated in foo's init() function (2)
> + * by one of the available implementations.  To call functions, use
> + * alternative_{,v}call() referencing the main ops object (3).
> + */
> +
> +#ifdef CONFIG_ALTERNATIVE_CALL
> +
> +#include <asm/alternative.h>
> +
> +#define __alt_call_maybe_initdata __initdata

My v3 comment here was:

"I think it wants (needs) clarifying that this may only be used if
 the ops object is used exclusively in alternative_{,v}call()
 instances (besides the original assignments to it, of course)."

I realize this was slightly too strict, as other uses from .init.*
are of course also okay, but I continue to think that - in
particular with the example using it - there should be a warning
about this possible pitfall. Or am I merely unable to spot the
wording change somewhere in the comment?

Jan