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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.07.2021 10:36, Andrew Cooper wrote:
> On 13/07/2021 07:28, Jan Beulich wrote:
>> 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).
> 
> Once alternatives have run, the ops structure is no longer referenced by
> anything at runtime, and can be reclaimed.

Oh, right - if all pointers have been consumed for patching, initdata
is of course fine.

Jan

> All the x86 examples are weird because we've either got extra data
> fields which are referenced at runtime, or we've not accelerated all
> function pointers.
> 
> In neither case does modifying an accelerated function pointer after the
> fact do what the programmer expected, and conditionally initdata makes
> this far more obvious.
> 
> ~Andrew
>