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

[Tim Deegan <tim@xxxxxxx>]

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. 

Cheers,

Tim.

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