Re: [PATCH] KVM: nVMX: Fix L2 guest hang if shadow page tables on EPT

[Paolo Bonzini <pbonzini@redhat.com>]

On 17/03/2017 18:28, Ladi Prosek wrote:
> On Fri, Mar 17, 2017 at 3:41 PM, Wanpeng Li <kernellwp@gmail.com> wrote:
>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>>
>> The L2 guest hang if shadow page tables on EPT, the trace on L1 shows that
>> L2 kvm_exit reason EXCEPTION_NMI and page fault repeatedly:
>>
>> qemu-system-x86-2821  [003] d..2    45.848814: kvm_entry: vcpu 0
>> qemu-system-x86-2821  [003] ...1    45.848827: kvm_exit: reason EXCEPTION_NMI rip 0xe05b info fe05b 80000b0e
>> qemu-system-x86-2821  [003] ...1    45.848827: kvm_page_fault: address fe05b error_code 14
>>
>> Commit 7ca29de21362 (KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT)
>> prevents to load L2's PDPTRs according to dereferencing L2's CR3 since it is
>> uninitialized in real mode. Hyper-V L1 will emulate L2 real mode with PAE
>> paging and EPT enabled. However, there is a progress to switch from Legacy
>> mode's such-mode Protected mode to Long mode during system boot, the check
>> in nested_vmx_load_cr3() will prevent to load PDPTRs if it is still in
>> Protected mode w/ PAE paging and nested EPT/shadow page tables on EPT. Actually
>> the original commit should just intended to prevent to dereference L2's CR3
>> if the L1 hypervisor emulates L2's real mode through vm8086.
>>
>> This patch fixes it by allowing load PDPTRs if PAE paing, EPT enabled and
>> !vm86_active.
>>
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Cc: Ladi Prosek <lprosek@redhat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
>> ---
>>  arch/x86/kvm/vmx.c | 4 ++--
>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
>> index c664365..2b2a05f 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -9933,7 +9933,7 @@ static bool nested_cr3_valid(struct kvm_vcpu *vcpu, unsigned long val)
>>  static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool nested_ept,
>>                                u32 *entry_failure_code)
>>  {
>> -       if (cr3 != kvm_read_cr3(vcpu) || (!nested_ept && pdptrs_changed(vcpu))) {
>> +       if (cr3 != kvm_read_cr3(vcpu) || pdptrs_changed(vcpu)) {
>>                 if (!nested_cr3_valid(vcpu, cr3)) {
>>                         *entry_failure_code = ENTRY_FAIL_DEFAULT;
>>                         return 1;
>> @@ -9944,7 +9944,7 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
>>                  * must not be dereferenced.
>>                  */
>>                 if (!is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu) &&
>> -                   !nested_ept) {
>> +                   !(nested_ept && to_vmx(vcpu)->rmode.vm86_active)) {
> 
> This change breaks Hyper-V on KVM. L2 hangs on start-up, same symptoms
> as before 7ca29de21362.

Looks like we need _two_ testcases then... :)

Paolo

> I'll take a closer look next week. Is there an easy way for me to
> reproduce the issue you're seeing?