[ltt-dev] [PATCH] fix the "unknown" case

[Ulrich Weigand]

Mathieu Desnoyers <mathieu.desnoyers at efficios.com> wrote on 06/15/2010
07:03:15 PM:

> I wonder starting with which Linux kernel version __kernel_dmb appeared.
> Tying ourself directly to a Linux kernel ABI might complicate things.
>
> Is this ABI presented in a vDSO or userland have to go through a system
call ?
> Is there any way to probe for its availability ?

This looks sort-of like a vDSO, except without the DSO part :-)

The kernel simply makes the code available at a fixed address that is
directly callable by user space.  See the comments in
linux/arch/arm/kernel/entry-armv.S:

/*
 * User helpers.
 *
 * These are segment of kernel provided user code reachable from user space
 * at a fixed address in kernel memory.  This is used to provide user space
 * with some operations which require kernel help because of unimplemented
 * native feature and/or instructions in many ARM CPUs. The idea is for
 * this code to be executed directly in user mode for best efficiency but
 * which is too intimate with the kernel counter part to be left to user
 * libraries.  In fact this code might even differ from one CPU to another
 * depending on the available  instruction set and restrictions like on
 * SMP systems.  In other words, the kernel reserves the right to change
 * this code as needed without warning. Only the entry points and their
 * results are guaranteed to be stable.
 *
 * Each segment is 32-byte aligned and will be moved to the top of the high
 * vector page.  New segments (if ever needed) must be added in front of
 * existing ones.  This mechanism should be used only for things that are
 * really small and justified, and not be abused freely.
 *
 * User space is expected to implement those things inline when optimizing
 * for a processor that has the necessary native support, but only if such
 * resulting binaries are already to be incompatible with earlier ARM
 * processors due to the use of unsupported instructions other than what
 * is provided here.  In other words don't make binaries unable to run on
 * earlier processors just for the sake of not using these kernel helpers
 * if your compiled code is not going to use the new instructions for other
 * purpose.
 */


/*
 * Reference prototype:
 *
 *           void __kernel_memory_barrier(void)
 *
 * Input:
 *
 *           lr = return address
 *
 * Output:
 *
 *           none
 *
 * Clobbered:
 *
 *           none
 *
 * Definition and user space usage example:
 *
 *           typedef void (__kernel_dmb_t)(void);
 *           #define __kernel_dmb (*(__kernel_dmb_t *)0xffff0fa0)
 *
 * Apply any needed memory barrier to preserve consistency with data
modified
 * manually and __kuser_cmpxchg usage.
 *
 * This could be used as follows:
 *
 * #define __kernel_dmb() \
 *         asm volatile ( "mov r0, #0xffff0fff; mov lr, pc; sub pc, r0,
#95" \
 *		         : : : "r0", "lr","cc" )
 */


As far as I can see, the only provision to check whether a feature is
available
is this one:

/*
 * Reference declaration:
 *
 *           extern unsigned int __kernel_helper_version;
 *
 * Definition and user space usage example:
 *
 *           #define __kernel_helper_version (*(unsigned int *)0xffff0ffc)
 *
 * User space may read this to determine the curent number of helpers
 * available.
 */

However, note that libgcc code does not perform this check, it simply
assumes
the above routine to be present.

The __kernel_dmb (which is the most recently added helper available in
current
mainline) seems to have been available since kernel 2.6.15, so presumably
code
using any of the GCC sync primitives would simply fail on any older kernel,
unless I'm missing something here ...


Mit freundlichen Gruessen / Best Regards

Ulrich Weigand

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