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

[weiming <zephyr.zhao@xxxxxxxxx>]

Hi Daniel,

Now, I'm asking for more. :}

1) As you said, in 64-bit mode, PT switch is needed. But on your other post, you said:

"So instead one lets a process, its guest kernel and the VMM share one

virtual address space. As with regular OSes: one address space per guest
process. Control transfers between process, kernel and VMM does not
switch between virtual memories, but only privileges, stacks, and EIP.
-> much faster."

The statements above  implies that PT switch is not needed.  Is it true?

2) in 0/3/3 mode, how guest kernel protects it from application? Simply use different page tables? Will it be too costly (need TLB flush)? Why not use FS, GS to implement 0/1/3 mode?

Many thanks!
Weiming

On Wed, Mar 12, 2008 at 5:37 AM, Daniel Stodden <stodden@xxxxxxxxxx> wrote:

On Tue, 2008-03-11 at 17:28 +0100, Daniel Stodden wrote:
> yes. The ring1 kernel only works if you can play some tricks on virtual
> memory management in order to protect ring1 kernel memory from ring3
> user space (paging alone only distinguishes 0 from 1-3). On x86_32,

i should probably correct myself. wrong story, sorry.

of course, paging distingushes ring 0-2 from 3.

so 32-bit in 0/1/3 with segmentation protects xen from the kernel, and
32-bit paging then protects xen and the kernel from user mode.

64-bit paging in 0/3/3 protects xen from both kernel and user mode, but
as i mentioned, you need a PT switch to keep the kernel safe.

maybe more than you asked for :}

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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