Force Linux to use only memory over 4G?

Question

I have a Linux device driver that interfaces to a device that, in theory, can perform DMA using 64-bit addresses. I\'d like to test to see that this actually works.

Answer 1

/usr/src/linux/Documentation/kernel-parameters.txt

        memmap=exactmap [KNL,X86] Enable setting of an exact
                        E820 memory map, as specified by the user.
                        Such memmap=exactmap lines can be constructed based on
                        BIOS output or other requirements. See the memmap=nn@ss
                        option description.

        memmap=nn[KMG]@ss[KMG]
                        [KNL] Force usage of a specific region of memory
                        Region of memory to be used, from ss to ss+nn.

        memmap=nn[KMG]#ss[KMG]
                        [KNL,ACPI] Mark specific memory as ACPI data.
                        Region of memory to be used, from ss to ss+nn.

        memmap=nn[KMG]$ss[KMG]
                        [KNL,ACPI] Mark specific memory as reserved.
                        Region of memory to be used, from ss to ss+nn.
                        Example: Exclude memory from 0x18690000-0x1869ffff
                                 memmap=64K$0x18690000
                                 or
                                 memmap=0x10000$0x18690000

If you add memmap=4G$0 to the kernel's boot parameters, the lower 4GB of physical memory will no longer be accessible. Also, your system will no longer boot... but some variation hereof (memmap=3584M$512M?) may allow for enough memory below 4GB for the system to boot but not enough that your driver's DMA buffers will be allocated there.