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[Xen-devel] One potential issue of shadow fault emulation

Currently shadow fault handler try to emulate up to four extra
instruction for PAE guest, to reduce vmexit times.

But there is a potential issue here: Consider the second instruction is
a change to virtual TPR register. In physical environment, if the TPR
acceleration is enabled, the cpu will try to access the
VIRTUAL_APIC_PAGE_ADDR set in the VMCS. However, when we do emulation,
we didn't cope with this situation, and will access the APIC_ACCESS_ADDR
page pointed by the shadow. This is sure cause problem to guest, usually
blue screen, and this issue will happen randomly depends on the content
in the  apic access page.

So how should we cope with such situation? Stop emulation or, continue
emulate , but access the virtual APIC page? Or any better idea?

-- Yunhong Jiang

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