Re: Question about physical page allocation to the guest

[Xiao Guangrong <xiaoguangrong.eric@gmail.com>]

On 10/20/14 2:09 AM, Steven wrote:
> Hi, Eric,
> I am trying to understand how KVM allocates physical pages to the
> guest and your slides clarify a lot of questions.
> (https://events.linuxfoundation.org/slides/2011/linuxcon-japan/lcj2011_guangrong.pdf)
>
> However, I still have some difficulty in figuring out what happens in
> the kvm code.
>
> The host kernel is 3.2.14 and EPT is disabled.
>
> In the guest VM, I run a micro-benchmark program that touches 1024
> pages (as an integer array). So in the guest VM, I can trace 1024
> mm_page_alloc event. However, in the hypervisor I can only trace about
> 45 (sometimes < 45) kvm_page_fault events, which means that most page
> faults in the guest are not exposed to the hypervisor. My questions
> are
>
> (1) why such kind of page faults are not exposed to the hypervisor as
> EPT is disabled? (My doubt is that it is related to non-presetn PTE as
> you discussed in the slides. But could you give some more details?)

Two cases, the one is the page you accessed have already mapped into the 
guest (the Present bit in the SPTE (shadow page table entry) is set). 
Another is that we can do page prefetch in KVM that can fill
more nonpresent sptes in one vm-exit.

>
> (2) In such case, when the physical page are allocated to the VM to
> backup the virtual pages? Could you give some hint about which piece
> of KVM code calling the get get_user_pages()?

In the case if the hva of the gpa you are accessing is not mapped in
the hypervisor's address space. Please follow the code of try_async_pf

>
> Thanks in advance.

Enjoy it. :)