Re: why xen use x86_emulation() in page_fault

["Zhengwang Ruan (Royo)" <zhengwang.ruan@gmail.com>]

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)?
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?

Thanks,
Royo (Zhengwang)


> Cheers,
>
> Tim.
>
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