[Bug runtime/12341] deref()/store_deref() variations between arches

[Bug runtime/12341] deref()/store_deref() variations between arches

[mjw at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12341

Mark Wielaard <mjw at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|ASSIGNED                    |NEW
         AssignedTo|mjw at redhat dot com       |systemtap at sourceware dot
                   |                            |org

--- Comment #5 from Mark Wielaard <mjw at redhat dot com> 2011-12-10 20:08:37 UTC ---
The deref/store_deref functions were split into user and kernel versions to
accommodate the s390x setup which uses different address spaces for user and
kernel addresses.

commit f1e8e7e0a601d1378262ba69890ce3e7d4519ab5
Author: Mark Wielaard <mjw@redhat.com>
Date:   Fri Nov 25 15:11:00 2011 +0100

    loc2c-runtime.h: Split deref and store_deref into user and kernel version.

    All references to deref have been updated to use either the user or kernel
    variant where appropriate, but kderef is currently still defined as uderef,
    and store_kderef as store_uderef. Which should work fine for all arches
    except s390.

There are still separate variants for the different architectures.

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[Bug runtime/12341] deref()/store_deref() variations between arches

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12341

David Smith <dsmith at redhat dot com> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |RESOLVED
         Resolution|                            |FIXED

--- Comment #6 from David Smith <dsmith at redhat dot com> 2012-02-06 20:48:08 UTC ---
Since all arches (except for arm) now use the kernel's own routines instead of
custom assembly code, let's close this one.

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[Bug runtime/12341] New: deref()/store_deref() variations between arches

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12341

           Summary: deref()/store_deref() variations between arches
           Product: systemtap
           Version: unspecified
            Status: NEW
          Severity: normal
          Priority: P2
         Component: runtime
        AssignedTo: systemtap@sources.redhat.com
        ReportedBy: dsmith@redhat.com


There is a different deref()/store_deref() macro in runtime/loc2c-runtime.h for
each arch.

For i386/x86_64, deref() calls the kernel's __get_user_asm().  store_deref()
calls the kernel's __put_user_asm().

For ia64, deref() calls the kernel's __get_user_size().  store_deref() calls
the kernel's __put_user_size().

For powerpc, deref() calls __stp_get_user_asm(), which is custom powerpc
assembly code (that looks almost identical to the kernel's __get_user_asm()). 
store_deref() calls __stp_put_user_asm(), which is custom powerpc assembly
code.

For s390, deref() calls __stp_get_user_asm(), which is custom s390 assembly. 
store_deref() calls __stp_put_user_asm(), which is custom s390 assembly.

It might be possible for all arches to use the kernel's own routines, instead
of having custom (powerpc/s390) assembly code.  The
__get_user_size()/__put_user_size() routines might be the most attractive
choice (as the ia64 uses), since they know how handle all the different size
combinations.

(If __get_user_size()/__put_user_size() works for the i386, that could
eliminate the need for the i386 special version of kread/kwrite that can handle
64-bit values.)

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[Bug runtime/12341] deref()/store_deref() variations between arches

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12341

--- Comment #1 from David Smith <dsmith at redhat dot com> 2011-01-04 21:50:16 UTC ---
(In reply to comment #0)
> (If __get_user_size()/__put_user_size() works for the i386, that could
> eliminate the need for the i386 special version of kread/kwrite that can handle
> 64-bit values.)

Unfortunately, this idea won't work.  Looking at the i386 header files in
kernels 2.6.9/2.6.18/2.6.36, none of the i386 __get_user_size() macros can
handle 64-bit values.

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[Bug runtime/12341] deref()/store_deref() variations between arches

[dsmith at redhat dot com]

http://sourceware.org/bugzilla/show_bug.cgi?id=12341

--- Comment #2 from David Smith <dsmith at redhat dot com> 2011-01-06 20:58:39 UTC ---
(In reply to comment #0)
> For powerpc, deref() calls __stp_get_user_asm(), which is custom powerpc
> assembly code (that looks almost identical to the kernel's __get_user_asm()). 
> store_deref() calls __stp_put_user_asm(), which is custom powerpc assembly
> code.

For powerpc, I've discovered some history.

loc2c-runtime.h originally used the kernel's __get_user_asm() macros.

This changed on 20 Mar 2006 in the following commit:

<http://sources.redhat.com/git/gitweb.cgi?p=systemtap.git;a=commitdiff;h=15eeafe9465119eb443f59b332e24ecfeec10488>

To fix bug 2076 (ppc64 runtime is broken under kernel v2.6.15-RC5):

<http://sourceware.org/bugzilla/show_bug.cgi?id=2076>

which was caused by the "__get_user_asm" macro changing to taking 4 arguments
(from 5).

There is (hopefully) a better way to fix that problem than duplicating
__get_user_asm()/__put_user_asm().

Note that we just can't go back to calling __get_user_asm(), since in current
kernels __get_user_asm2() is used on powerpc to get 64-bit values.  We'll need
to call something a bit higher up.

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