Can I allocate memory pages at a specified physical address in a kernel module?

Question

I am writing a kernel module in a guest operating system that will be run on a virtual machine using KVM. Here I want to allcoate a memory page at a particular physical address. kmalloc() gives me memory but at a physical address chosen by the OS.

Background : I am writing a device emulation technique in qemu that wouldn't exit when the guest communicates with the device (It exits, for example, in I/O mapped as well as port mapped devices). The basic idea is as follows : The guest device driver will write to a specific (guest) physical memory address. A thread in the qemu process will be polling it continuously to check for new data (through some status bits etc.). And will take action accordingly without causing an exit. Since there is no (existing) way by which guest can tell the host what address is being used by the device driver, I want a pre-specified memory page to be allocated for it.

There is the way. It is a DMA with emulated-by-qemu hardware. There are device drivers in guest OS which will setup DMA via "PCI" or IO Ports and there are emulated hardware devices in qemu (like hdd, network). — osgx
@osgx Yes, but each access to the memory location by the guest device driver would cause an exit to qemu. I am trying to minimize these exits to improve device emulation performance — AnkurVj

Dan Aloni Dan Aloni · Accepted Answer · 2011-11-09T22:37:08

You cannot allocate memory at a specific address, however, you can reserve certain physical addresses on boot time using reserve_bootmem(). Calling reserve_bootmem() early on boot (of course, it requires a modified kernel) will ensure that the reserved memory will not be passed on to the buddy system (i.e. alloc_pages() and higher level friends - kmalloc()), and you will be able to use that memory for any purpose.

Can I allocate memory pages at a specified physical address in a kernel module?

2 Answers