Re: [Qemu-devel] About qemu emulation speed (a question) and supported OS

[Henrik Nordstrom]

On Wed, 14 Sep 2005, Jim C. Brown wrote:

> Not familar with L4ka. I don't believe that UML does virtualization, it simply
> runs linux code 'as is' but intercepts calls to the kernel.

UML does not do hardware virtualization. UML is a special architecture for 
the Linux kernel allowing Linux to run ontop of Linux. It does so by using 
it's own drivers and memory management, intercepting Linux system calls 
and playing with the page tables. All executed code is native in ring 3 
(even the kernel parts, just a different memory mapping when the kernel 
code executes). The UML Linux kernel does not access any hardware, instead 
it provides drivers for "simulated" harddrives, serial ports etc 
translating these into suitable user-space actions on the host (i.e. 
read/writes to a block device is translated to read/writes in a file on 
the host, etc.. at the kernel driver level, not by hardware 
simulation/emulation/partitioning).

The UML Linux kernel is running as a normal application on the host. But 
within the process virtual memory maps is provided for user-space 
applications and any systems calls made by user-space applications running 
there is intercepted with help from the syscall tracing facilities of the 
host and instead acted on by the UML Linux kernel.

In theory UML should be able to run at native speed as there is no 
virtualization, translation or emulation involved, but as always there is 
overhead from various sources. To gain better performance the UML people 
is playing with

  a) More efficient management of multiple page tables for the same host 
process, allowing efficient switches between different contexts of the 
same process (kernel mode, user-space processes within UML etc).

  b) More efficient methods intercepting system calls.

  c) more efficient I/O drivers, mainly for block device access.

Regards
Henrik