linux-kernel - Re: [PATCH v2 03/13] powerpc: Remove func_descr

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Date:   Fri, 15 Oct 2021 16:11:32 +1000
From:   Nicholas Piggin <npiggin@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>,
        Arnd Bergmann <arnd@...db.de>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        Helge Deller <deller@....de>, Daniel Axtens <dja@...ens.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "James E.J. Bottomley" <James.Bottomley@...senPartnership.com>,
        Kees Cook <keescook@...omium.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Paul Mackerras <paulus@...ba.org>
Cc:     linux-arch@...r.kernel.org, linux-ia64@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-parisc@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH v2 03/13] powerpc: Remove func_descr_t

Excerpts from Christophe Leroy's message of October 15, 2021 3:19 pm:
> 
> 
> Le 15/10/2021 à 00:17, Daniel Axtens a écrit :
>> Christophe Leroy <christophe.leroy@...roup.eu> writes:
>> 
>>> 'func_descr_t' is redundant with 'struct ppc64_opd_entry'
>> 
>> So, if I understand the overall direction of the series, you're
>> consolidating powerpc around one single type for function descriptors,
>> and then you're creating a generic typedef so that generic code can
>> always do ((func_desc_t)x)->addr to get the address of a function out of
>> a function descriptor regardless of arch. (And regardless of whether the
>> arch uses function descriptors or not.)
> 
> An architecture not using function descriptors won't do much with 
> ((func_desc_t *)x)->addr. This is just done to allow building stuff 
> regardless.
> 
> I prefer something like
> 
> 	if (have_function_descriptors())
> 		addr = (func_desc_t *)ptr)->addr;
> 	else
> 		addr = ptr;

If you make a generic data type for architectures without function 
descriptors as such

typedef struct func_desc {
    char addr[0];
} func_desc_t;

Then you can do that with no if. The downside is your addr has to be 
char * and it's maybe not helpful to be so "clever".

>>   - why pick ppc64_opd_entry over func_descr_t?
> 
> Good question. At the begining it was because it was in UAPI headers, 
> and also because it was the one used in our 
> dereference_function_descriptor().
> 
> But at the end maybe that's not the more logical choice. I need to look 
> a bit more.

I would prefer the func_descr_t (with 'toc' and 'env') if you're going 
to change it.

Thanks,
Nick