linux-kernel - Re: [PATCH 14/22] parisc: %pF is only for function pointers

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Message-ID: <BLU437-SMTP4321320765AD407013D80497060@phx.gbl>
Date:	Thu, 12 Mar 2015 10:43:51 -0400
From:	John David Anglin <dave.anglin@...l.net>
To:	James Bottomley <James.Bottomley@...senPartnership.com>,
	Scott Wood <scottwood@...escale.com>
CC:	trivial@...nel.org, linux-kernel@...r.kernel.org,
	linux-parisc@...r.kernel.org
Subject: Re: [PATCH 14/22] parisc: %pF is only for function pointers

On 2015-03-12 8:11 AM, James Bottomley wrote:
> On Wed, 2015-03-11 at 22:13 -0500, Scott Wood wrote:
>> Use %pS for actual addresses, otherwise you'll get bad output
>> on arches like ppc64 where %pF expects a function descriptor.  Even on
>> other architectures, refrain from setting a bad example that people
>> copy.
> Are you sure about this?  Parisc64 is a function description
> architecture.  There may be a misunderstanding about what
> __builtin_return_address(0) is supposed to return, but I'm certain the
> person who added the code thought it returned a function pointer, which
> on parisc64 would be a descriptor.

__builtin_return_address(0) returns the return address in the calling procedure
ignoring import/export stubs.  There are no function descriptors for return addresses.
Thus, it can't return a function pointer.

There are no function descriptors in 32-bit parisc when the -mfast-indirect-calls
compiler option is used.

This option used to be used for 64-bit kernel builds but this broke when the
-mfast-indirect-calls was fixed for user space (gcl uses it).  I worked a bit
on trying to eliminate function descriptors from the 64-bit kernel for performance
but I don't have a working change.

Dave

-- 
John David Anglin  dave.anglin@...l.net

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