Re: [Xen-devel] how page faults are handled in paravirtualized xenguests?

[weiming <zephyr.zhao@xxxxxxxxx>]

Hi Daniel & Haifeng,

Thanks for your elaboration. 
Now I'm almost clear on this problem.

Daniel, Could you please explain why "not the case for 64-bit systems."
For system calls, a regular OS will issure INT 0x80 to do system calls. In Xen, this interrupt will be handled by the guest directly. In guest OS can use 0x82 interrupt to call hypercalls.

So how is x86_64 different? I'm more interested in 64-bit system because I'll work on it.

Thanks a lot!
Weiming

On Sun, Mar 9, 2008 at 10:33 AM, Daniel Stodden <stodden@xxxxxxxxxx> wrote:

On Sun, 2008-03-09 at 10:27 +0800, 房海峰 wrote:
> hello, my friend:
>
> According to my research with source code of xen, i think the process
> is that:
>
> first, when Guest OS is created by Xen/Domian0, it install it's IDT
> through "set_trap_table" hypercall.
> second, during Xen/GuestOS running, all interrupt/trap will result a
> trap into Ring0, that is the layer of Xen.
> for some trap, such as system-call, they will be dealt with directly
> through interrupt hardware.
> For the most of other traps, they will be dealt by xen, and then
> dispatched to corresponding Guest OS.
>
> in addition, there are other details to study in-depth.

Generally, the IDT entry may either point to xen or directly to the
trap handler installed by the guest.

For system calls, it presently depends on the architecture: x86_32 has
separate trap instructions for hypercalls and system calls, which can be
forwarded directly. But is not the case for 64-bit systems.

In fault handling, it depends on the type of fault. It may be either due
to the virtualization layer, then transparently fixed by Xen (e.g.
instruction emulation). Or the guest may be responsible (e.g. a process
page fault). In that case the fault will be forwarded.

Some faults (e.g. division by zero) are indeed never Xen's business.
Still, you'll find the idt entry to point to xen, instead of directly to
the ring1 kernel. See the use of DO_ERROR_*() in x86/traps.c. The
reason is simply that bugs in Xen (of course, that's impossible :), but
you never know) would not be caught otherwise.

So, the bottom line is that all goes through Xen, except for 32-on-32
bit system calls.

hth,
daniel

--
Daniel Stodden
LRR     -      Lehrstuhl für Rechnertechnik und Rechnerorganisation
Institut für Informatik der TU München             D-85748 Garching
http://www.lrr.in.tum.de/~stodden         mailto:stodden@xxxxxxxxxx
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