Behavior of procnto-v6 shm_ctl()

The ARMv6 procnto-v6 removes the 32 MB process address space limit:

• The per-process address space is now 2 GB.
• There is a limit of 256 address spaces imposed by the MMU ASID register.

The procnto-v6 microkernel doesn't implement the ARM-specific global memory region implemented by the non-ARMv6 procnto. This means that shm_ctl() no longer has any ARM-specific special behavior. The shm_ctl() function exhibits the following:

• Objects created with shm_ctl() are always mapped into the per-process address space. These mappings are inherited across a fork() function.

  In contrast, the non-ARMv6 behavior causes some mappings to be placed in the global memory region, and these global mappings aren't inherited across a fork() function.

• The SHMCTL_PHYS flag maps the specified physical address range.

  This differs from the non-ARMv6 procnto behavior, where the mapping is placed in the global address space to allow mappings that would not otherwise fit into the 32 MB per-process address space.

• The SHMCTL_ANON maps anonymous memory. This is identical to the non-ARMv6 procnto behavior.
• The SHMCTL_ANON|SHMCTL_PHYS flag maps physically contiguous anonymous memory. This is identical to the non-ARMv6 procnto behavior.
• The SHMCTL_GLOBAL flag is ignored, since all mappings are placed in the per-process address space.

  In order to achieve the same effect as the non-ARMv6 SHMCTL_GLOBAL, it is necessary to assign a known address to the mapping, then use MAP_FIXED to mmap() the shared memory object at the same address in each process. You can do this in a number of ways:

  • The first process to map uses a regular mmap() where the system assigns the virtual address. In this case, the address must be communicated in some way to other processes that wish to map the object. These subsequent mappings must use MAP_FIXED.
  • The mapping is assigned a predefined virtual address that is used by all processes by using MAP_FIXED.
• The SHMCTL_PRIV and SHMCTL_LOWERPROT flags are ignored.

  All mappings are created in the per-process address space with user mode access protections.

  For the non-ARMv6 procnto, these flags are used to optimize performance by indicating that the mapping doesn't require updating the global memory page tables on context switches to implement per-process protection.

If code must run on both ARMv6 and non-ARMv6 processors, you must check the __cpu_flags value at runtime to select the correct implementation. For example:

if (__cpu_flags & ARM_CPU_FLAG_V6) {
   /*
   * Code for ARMv6 processor only
   */
   } else {
     /*
     * Code for non-ARMv6 processor only
     */
     }