[Qemu-devel] nested page table translation for non-x86 operating system

[Xin Tong]

I am wondering the possibilities of using the nested page table
mechanism available on the x86 processors to do page translation for
non-x86 operating system emulation.

So, when nested page is enabled, you can control the gCR3 and hCR3.
The gCR3 can be used to point to the page table of the running process
in the guest operating system and the hCR3 can be used to point to the
page table of the QEMU process. Assuming the page table layouts of
both operating systems are exactly the same.  I think this can be
done. However there are a few problems I see here. I would like to
hear some suggestions or corrections.

1.  The control of gCR3 and hCR3 needs kernel access. While they can
be set with a device module as what is done in kvm. Trapping into the
kernel every time gCR3 is reseted might be too expensive.

2. After setting the gCR3 and hCR3. whatever memory references fall
within the guest memory will be done correctly. However, memory
references done by the host will be broken. Therefore, when we load
the from the CPUstates, call to helpers for exits from the code cache,
we need to change the paging mechanism back to non-nested. can this be
done ? how expensive will this be ?

3. Lastly and most importantly,  the code cache is based on a host
address, what about fetching instructions from the code cache, this
has to happen in non-nested mode ?


Thanks


Xin