Re: [PATCH v2 01/10] xen: Implement xen/alternative-call.h for use in common code

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.07.2021 01:48, Andrew Cooper wrote:
> On 12/07/2021 21:32, Daniel P. Smith wrote:
>> diff --git a/xen/include/xen/alternative-call.h 
>> b/xen/include/xen/alternative-call.h
>> new file mode 100644
>> index 0000000000..11d1c26068
>> --- /dev/null
>> +++ b/xen/include/xen/alternative-call.h
>> @@ -0,0 +1,65 @@
>> +/* 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) struct foo_ops __alt_call_maybe_initconst foo_a_ops = { ... };
>> + *     struct foo_ops __alt_call_maybe_initconst foo_b_ops = { ... };
> 
> It occurs to me after reviewing patch 2 that these want to be const
> struct foo_ops __initconst ...,

__initconstrel then, I suppose.

> and there is no need for
> __alt_call_maybe_initconst at all.
> 
> The only thing wanting a conditional annotation like this is the single
> ops object, and it needs to be initdata (or hopefully ro_after_init in
> the future).

ro_after_init and initdata can't be alternatives of one another; ops
(until be gain ro_after_init) need to remain in .bss (or .data).

Jan