Re: Nested paging in nested SVM setup

[Valentine Sinitsyn <valentine.sinitsyn@gmail.com>]

Hi Jan,

> If we want to provide useful nested SVM support, this must be feasible.
> If there is a bug, it has to be fixed.
I was more concerned about if it is supported (and it means I do 
something wrong), or if it is not supported (at least, now).

> Maybe you can describe how you configured the involved units (NPT
> structures, guest / host PAR, MTRR etc.).
I've tried different combinations, but to be specific:
- NPT: four-level long-mode page tables; all PTEs except terminal have 
U,R,P bits set (0x07), as per APMv2 15.25.5
- APIC page pte; physical address 0xfee00000, flags: PAT, PWT, PCD, U, P 
(0x9D)
- guest PAT and host PAT are the same, 0x7010600070106 (as set by the 
Linux kernel). Guest PAT is stored in VMCB; host PAT is restored at each 
#VMEXIT.
- MTRRs. No changes to what Linux use prior to VM entry here; #0 (base 
0x80000000, mask 0xFF80000000) uncacheable, others are disabled.

I also noticed that setting PAT MSR from the nested hypervisor leaves 
high word unassigned, i.e. the code like this:

   mov $0x70106, %rax
   mov %rax, %rdx
   mov $0x0277, %rcx
   wrmsr
   rdmsr

yields %rax = 0, %rdx = 0x70106.

> Even better would be a test case based on kvm-unit-tests (see [1],
Will have a look at it, thanks.

--
Best regards,
Valentine Sinitsyn