Re: [Xen-devel] why xen use x86_emulation() in page_fault

[cc Luit <universalbillow@xxxxxxxxx>]

On Tue, Oct 11, 2011 at 8:04 PM, Zhengwang Ruan (Royo) <zhengwang.ruan@xxxxxxxxx> wrote:

Hi Tim,

I am a freshman to this member-list and interested in some contents in your reply, looking forward to your answer. :)

At 2011-10-11 16:18, Tim Deegan wrote:

At 09:39 +0800 on 11 Oct (1318325957), cc Luit wrote:

Hi, everyone, I have a question,
in the shadow_page_fault or ept mechanism, xen will use the x86_emulation
for some instructions, I'm wondering why it must use it, if after we fix the
SPT or EPT table, just VMEntry to HVM to re-excute this instruction  but not
emulate in xen, is there some problems?

In the shadow pagetable code, we keep the shadows up-to-date by:
 1 - making all shadowed pagetables read-only;
 2 - intercepting the page faults when the guest writes to them; and
 3 - updating the guest pagetable and the shadow at the same time,
     with whatever change the guest was making.

For step 3 we need to emulate the instruction that caused the pagefault
so that we can tell what was being written.

There are other reasons for the emulator to be called (emulating MMIO

instructions, emulating real-mode&c) but that's why the shadow
pagetable code uses it.

1, Does this emulator set up a emulation environment to execute instructions for a VM(guest OS)?

there is a x86_emulate() in xen/arch/x86/x86_emulate/x86_emulate.c(my version is xen-4.0.1), and it will emulate some instructions (I think both guest OS and Domain-0) in some conditions(like what Tim said)

 

2, Real-mode code you talked above is used to execute the first 16-bit initial code of a guest OS? Does it have other usages in current Xen implementation?

I'm also curious about the real-mode, looking forward to the replay~

Thanks,
Royo (Zhengwang)

Cheers,

Tim.

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-- 
- Luit @ Parallel Processing Institute, Fudan University 
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