Re: Ping: [PATCH v2] x86/time: switch platform timer hooks to altcall

[Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>]

On 24/02/2022 11:25, Jan Beulich wrote:
> On 13.01.2022 14:17, Jan Beulich wrote:
>> Except in the "clocksource=tsc" case we can replace the indirect calls
>> involved in accessing the platform timers by direct ones, as they get
>> established once and never changed. To also cover the "tsc" case, invoke
>> what read_tsc() resolves to directly. In turn read_tsc() then becomes
>> unreachable and hence can move to .init.*.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> As this actually amends the IBT work, I would have hoped for it to be
> viewed as useful.

Sorry - it fell through the cracks.  Definitely useful.

>  Of course if accepted in general, it would now want
> to have __initconst_cf_clobber annotation addition included. Albeit
> there's a slight complication: Some of the structures are written to,
> so those couldn't really be "const".

The .init.cf_clobber section needs to container a pointer to every
target function.  For the current ops structures, we just put the whole
ops structure in.

For individual functions, the best plan I could come up with was a macro
which emits:

.pushsection .init.cf_clobber, a, @progbits
.quad fn
.popsection

wrapped up in #define cf_clobber(fn), so the end code result ought to
look like:

static void foo(param *bar)
{
    ...
}
cf_clobber(foo);

similar to command line parameters.


That said, in this case, can't we cf_clobber each platform_timesource ? 
It would require altcall()ing the resume hook too.  (the init hook won't
matter either way.)

~Andrew