Re: Nested paging in nested SVM setup

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

Hi all,

Please excuse me for bringing alive a two-month old thread, but I had 
time to investigate the issue a bit only recently.

On 18.06.2014 18:47, Jan Kiszka wrote:
> On 2014-06-18 13:36, Valentine Sinitsyn wrote:
> If we want to provide useful nested SVM support, this must be feasible.
> If there is a bug, it has to be fixed.
Looks like it is a bug in KVM. I had a chance to run the same code 
bare-metal ([1], line 310 is uncommented for bare-metal case but present 
for nested SVM), and it seems to work as expected. However, When I trace 
it in nested SVM setup, after some successful APIC reads and writes, I 
get the following:

>  qemu-system-x86-1968  [001] 220417.681261: kvm_nested_vmexit:    rip: 0xffffffff8104f5b8 reason: npf ext_inf1: 0x000000010000000f ext_inf2: 0x00000000fee00300 ext_int:
>  0x00000000 ext_int_err: 0x00000000
>  qemu-system-x86-1968  [001] 220417.681261: kvm_page_fault:       address fee00300 error_code f
>  qemu-system-x86-1968  [001] 220417.681263: kvm_emulate_insn:     0:ffffffff8104f5b8:89 04 25 00 93 5f ff (prot64)
>  qemu-system-x86-1968  [001] 220417.681268: kvm_inj_exception:     (0x23c)
>  qemu-system-x86-1968  [001] 220417.681269: kvm_entry:            vcpu 0
>  qemu-system-x86-1968  [001] 220417.681271: kvm_exit:             reason  rip 0xffffffff8104f5b8 info 0 0

You can see the problem here: the code tries to access APIC MMIO 
register, which is trapped by KVM's MMU code (at nested page table 
walk). During MMIO access emulation, KVM decides to inject 0x23c 
exception (which looks wrong, as there is no exception with this number 
defined). After that things become flawed (pay attention to empty reason 
in the last line; the VMCB is certainly not in the state KVM 
expects/supports).

I'm no KVM expert, and will be grateful for debugging suggestions (or 
maybe even assistance).

Many thanks for the help.

1. 
https://github.com/vsinitsyn/jailhouse/blob/amd-v/hypervisor/arch/x86/svm.c#L301

--
Regards,
Valentine Sinitsyn