Gleb Natapov wrote on 2013-08-06:
> On Tue, Aug 06, 2013 at 10:39:59AM +0200, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>>
>> If nested EPT is enabled, the L2 guest may change CR3 without any exits.
>> We therefore have to read the current value from the VMCS when
>> switching to L1. However, if paging wasn't enabled, L0 tracks L2's
>> CR3, and
>> GUEST_CR3 rather contains the real-mode identity map. So we need to
>> retrieve CR3 from the architectural state after conditionally
>> updating it - and this is what kvm_read_cr3 does.
>>
> I have a headache from trying to think about it already, but shouldn't
> L1 be the one who setups identity map for L2? I traced what
> vmcs_read64(GUEST_CR3)/kvm_read_cr3(vcpu) return here and do not see
Here is my understanding:
In vmx_set_cr3(), if enabled ept, it will check whether target vcpu is enabling paging. When L2 running in real mode, then target vcpu is not enabling paging and it will use L0's identity map for L2. If you read GUEST_CR3 from VMCS, then you may get the L2's identity map not L1's.
> different values in real mode.
>
>> Signed-off-by: Jan Kiszka <jan.kiszka@xxxxxxxxxxx>
>> ---
>> arch/x86/kvm/vmx.c | 2 +-
>> 1 files changed, 1 insertions(+), 1 deletions(-)
>> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c index
>> b482d47..09666aa 100644
>> --- a/arch/x86/kvm/vmx.c
>> +++ b/arch/x86/kvm/vmx.c
>> @@ -8106,7 +8106,7 @@ static void prepare_vmcs12(struct kvm_vcpu
> *vcpu, struct vmcs12 *vmcs12)
>> * Additionally, restore L2's PDPTR to vmcs12.
>> */
>> if (enable_ept) {
>> - vmcs12->guest_cr3 = vmcs_read64(GUEST_CR3);
>> + vmcs12->guest_cr3 = kvm_read_cr3(vcpu);
>> vmcs12->guest_pdptr0 = vmcs_read64(GUEST_PDPTR0);
>> vmcs12->guest_pdptr1 = vmcs_read64(GUEST_PDPTR1);
>> vmcs12->guest_pdptr2 = vmcs_read64(GUEST_PDPTR2);
>> --
>> 1.7.3.4
>
Best regards,
Yang
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