[Xen-devel] Question about page table used by hypervisor

[KennyChen <windtracekimo@xxxxxxxxxxxx>]

Hi, Tim:

One more doubt here.
In the Linux concept, each process all share a common part of kernel page
tables, e.g., 3-4GB address mapping are the same among all processes. Thus when
switching to kernel, there is no need to change hardware CR3.

But when going to PV guest, is that the same way?
I thought Xen prepares the page tables for PV guest in such a way that some
portion of address mapping is reserved particularly for hypervisor, which can be
used for translation inside hypervisor.

For PV, it might be ok because the page table is directly translated to machine
physical address. So when switching to hypervisor, this page table (same cr3)
can still be used for address translation.

But how about HVM guest (assume EPT is enabled)?
When vmexit happens, does the hardware CR3 switches to some other value, which
points to a special page table for Xen's use? If no switching happens, then the
hardware MMU would walk through guest page table for translation which is weird
to me.

If such special page table exists, could you point it out?

Thanks,
Kenny




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