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Asked: 2026-05-13T05:39:09+00:00

I have a Linux device driver that interfaces to a device that, in theory,

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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.

Is there a simple way that I can force a Linux machine not to use any memory below physical address 4G? It’s OK if the kernel image is in low memory; I just want to be able to force a situation where I know all my dynamically allocated buffers, and any kernel or user buffers allocated for me are not addressable in 32 bits. This is a little brute force, but would be more comprehensive than anything else I can think of.

This should help me catch (1) hardware that wasn’t configured correctly or loaded with the full address (or is just plain broken) as well as (2) accidental and unnecessary use of bounce buffers (because there’s nowhere to bounce to).

clarification: I’m running x86_64, so I don’t care about most of the old 32-bit addressing issues. I just want to test that a driver can correctly interface with multitudes of buffers using 64-bit physical addresses.

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1 Answer

  1. Editorial Team

    /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.

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