[vbox-dev] Fwd: Explanation about Ring0 context

Frank Mehnert frank.mehnert at oracle.com
Tue Feb 2 09:18:34 UTC 2016

Hi Luca,

device emulation code in VirtualBox can run within three contexts:

* R3 (part of VBoxDD): Normal userland code executed in the VM
  process context. This code is executed each time we leave the
  guest and go back to userland. This code is not that performance-
  critical, e.g. device initialization, memory allocation etc.

* R0 (part of VMMR0): Code which is executed in kernel context.
  This happens if the VM runs in VT-x/AMD-V mode and we left the
  VM and entered the root mode where the VirtualBox VMM runs
  (next to the host OS kernel). For performance reasons we don't
  switch to userland (R3). The amount of R0 code is much smaller
  than the amount of R3 code. Such code can also call host OS
  kernel functions directly (e.g. submit a network IP packet to
  the host OS network layer). Calling the host OS code from VMMR0
  is usually done using SUPR0* functions which are implemented in
  src/VBox/HostDrivers/Support and runtime functions which are
  implemented in src/VBox/Runtime/r0drv

* RC/GC (part of VMMRC.rc): This code is executed if the VM runs
  in non VT-x/AMD-V mode (legacy). Only 32-bit code. This code is
  part of the hypervisor which runs in R0 in the context of the
  guest process. The guest itself runs at R1 (guest userland as
  R3 as usual). Google should explain you x86 ring compression.

Of course R3 code cannot directly call R0 code. The code in our
device driver has sections which are unique to two or all three
contexts. That means that this code is compiled three times and
exists in all three contexts. Other code is exclusively used in
one or two contexts.

Kind regards,


On Tuesday 02 February 2016 10:00:55 Luca Carotenuto wrote:
> Gregory, thanks for the reply;
> anyway, my question was about R3 context of the host.
> Since the host must emulate the E1000 behaviour, it doesn't need to access
> the I/O space on the host. So, why does it need to be in R0 context?
> Also, as far as I know, the file VBoxDD.so is a library for R3 context, and
> it has the references
> for functions running in R0 context (e.g. e1kRegWriteTDT).
> How can it be possible?
> Kind Regards,
> Luca Carotenuto
> 2016-02-01 16:11 GMT+01:00 Gregory Woodbury <redwolfe at gmail.com>:
> > Depending on the CPU architecture, Ring 3 processes cannot access the
> > I/O space without causing a General Protection Exception.
> > 
> > Callback routines from IO requests are to let the application be told
> > that an operation is completed.
> > And thus have to be in the application's memory space, but they are
> > actually called from the
> > kernel in R0 space. There are special provisions in the x86 type
> > architecture for this, and
> > the callback routine has only a limited amount of freedom to access IO
> > space.
> > 
> > In a fully emulated environment, this might not be necessary, but
> > using the hardware virtualization of KVM/QEMU, to access
> > the IO space still requires Ring0 privleges.  I is a sort of
> > mind-bending set of restrictions and interactioins until one
> > gets used to thinking like the system developers did.
> > 
> > 
> > On Mon, Feb 1, 2016 at 6:59 AM, Luca Carotenuto
> > 
> > <luca.carotenuto.91 at gmail.com> wrote:
> > > 1) As I understand, when we talk about Ring-0 context, we refer to
> > > kernel
> > > space, while Ring-3 context refers to user space. Am I right?
> > > 
> > > 2) I'm looking inside the E1000 emulation code
> > > (/src/VBox/Devices/Network/DevE1000.cpp).
> > > Assuming that the above is right, for what concerns the registers
> > 
> > callbacks,
> > 
> > >  I do not understand why those callbacks are compiled to be executed in
> > > 
> > > Ring-0 context
> > > (When I use logging inside those functions, the thread name i R0).
> > > Infact, looking inside the VboxDD.so file in the "out/bin" directory
> > 
> > (using
> > 
> > > objdump tool), it seems that this code "belongs" to Ring-3 context,
> > > so why is it running in Ring-0 context?
> > 
> > --
> > G.Wolfe Woodbury
> > redwolfe at gmail.com

Dr.-Ing. Frank Mehnert | Software Development Director, VirtualBox
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