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

[Xin Tong]

On Fri, Jan 20, 2012 at 3:23 AM, 陳韋任 <address@hidden> wrote:
>> 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.
>
>  Why the control of gCR3 needs kernel access? Isn't gCR3 just a field of the
> CPUX86State? QEMU should have the control of it. Or you mean the trapping 
> thing?

I do not think gCR3 is a field in the CPUx86State. I think inorder to
change the guest CR3, we need to trap into the kernel as kvm does.
>
>> 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 ?
>
>  Why the memeory references done by the host will be broken?

the CPUstate is a host memory, if nested paging is enabled, the guest
page table is walked  and then the host. however, for memory accesses
to CPUstate, we do not want to guest page table to be walked.

>
> Regards,
> chenwj
>
> --
> Wei-Ren Chen (陳韋任)
> Computer Systems Lab, Institute of Information Science,
> Academia Sinica, Taiwan (R.O.C.)
> Tel:886-2-2788-3799 #1667
> Homepage: http://people.cs.nctu.edu.tw/~chenwj