Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST_CR3 during nested VM-Enter

[Liran Alon]

> On 27 Sep 2019, at 17:27, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
> 
> On Fri, Sep 27, 2019 at 03:06:02AM +0300, Liran Alon wrote:
>> 
>> 
>>> On 27 Sep 2019, at 0:43, Sean Christopherson <sean.j.christopherson@xxxxxxxxx> wrote:
>>> 
>>> Write the desired L2 CR3 into vmcs02.GUEST_CR3 during nested VM-Enter
>>> isntead of deferring the VMWRITE until vmx_set_cr3().  If the VMWRITE
>>> is deferred, then KVM can consume a stale vmcs02.GUEST_CR3 when it
>>> refreshes vmcs12->guest_cr3 during nested_vmx_vmexit() if the emulated
>>> VM-Exit occurs without actually entering L2, e.g. if the nested run
>>> is squashed because L2 is being put into HLT.
>> 
>> I would rephrase to âIf an emulated VMEntry is squashed because L1 sets
>> vmcs12->guest_activity_state to HLTâ.  I think itâs a bit more explicit.
>> 
>>> 
>>> In an ideal world where EPT *requires* unrestricted guest (and vice
>>> versa), VMX could handle CR3 similar to how it handles RSP and RIP,
>>> e.g. mark CR3 dirty and conditionally load it at vmx_vcpu_run().  But
>>> the unrestricted guest silliness complicates the dirty tracking logic
>>> to the point that explicitly handling vmcs02.GUEST_CR3 during nested
>>> VM-Enter is a simpler overall implementation.
>>> 
>>> Cc: stable@xxxxxxxxxxxxxxx
>>> Reported-by: Reto Buerki <reet@xxxxxxxxxxx>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
>>> ---
>>> arch/x86/kvm/vmx/nested.c | 8 ++++++++
>>> arch/x86/kvm/vmx/vmx.c    | 9 ++++++---
>>> 2 files changed, 14 insertions(+), 3 deletions(-)
>>> 
>>> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
>>> index 41abc62c9a8a..971a24134081 100644
>>> --- a/arch/x86/kvm/vmx/nested.c
>>> +++ b/arch/x86/kvm/vmx/nested.c
>>> @@ -2418,6 +2418,14 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>>> 				entry_failure_code))
>>> 		return -EINVAL;
>>> 
>>> +	/*
>>> +	 * Immediately write vmcs02.GUEST_CR3.  It will be propagated to vmcs12
>>> +	 * on nested VM-Exit, which can occur without actually running L2, e.g.
>>> +	 * if L2 is entering HLT state, and thus without hitting vmx_set_cr3().
>>> +	 */
>> 
>> If I understand correctly, itâs not exactly if L2 is entering HLT state in
>> general.  (E.g. issue doesnât occur if L2 runs HLT directly which is not
>> configured to be intercepted by vmcs12).  Itâs specifically when L1 enters L2
>> with a HLT guest-activity-state. I suggest rephrasing comment.
> 
> I deliberately worded the comment so that it remains valid if there are
> more conditions in the future that cause KVM to skip running L2.  What if
> I split the difference and make the changelog more explicit, but leave the
> comment as is?

I think what is confusing in comment is that it seems to also refer to the case
where L2 directly enters HLT state without L1 intercept. Which isnât related.
So I would explicitly mention itâs when L1 enters L2 but donât physically enter guest
with vmcs02 because L2 is in HLT state.

-Liran

> 
>>> +	if (enable_ept)
>>> +		vmcs_writel(GUEST_CR3, vmcs12->guest_cr3);
>>> +
>>> 	/* Late preparation of GUEST_PDPTRs now that EFER and CRs are set. */
>>> 	if (load_guest_pdptrs_vmcs12 && nested_cpu_has_ept(vmcs12) &&
>>> 	    is_pae_paging(vcpu)) {
>>> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
>>> index d4575ffb3cec..b530950a9c2b 100644
>>> --- a/arch/x86/kvm/vmx/vmx.c
>>> +++ b/arch/x86/kvm/vmx/vmx.c
>>> @@ -2985,6 +2985,7 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>>> {
>>> 	struct kvm *kvm = vcpu->kvm;
>>> 	unsigned long guest_cr3;
>>> +	bool skip_cr3 = false;
>>> 	u64 eptp;
>>> 
>>> 	guest_cr3 = cr3;
>>> @@ -3000,15 +3001,17 @@ void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3)
>>> 			spin_unlock(&to_kvm_vmx(kvm)->ept_pointer_lock);
>>> 		}
>>> 
>>> -		if (enable_unrestricted_guest || is_paging(vcpu) ||
>>> -		    is_guest_mode(vcpu))
>>> +		if (is_guest_mode(vcpu))
>>> +			skip_cr3 = true;
>>> +		else if (enable_unrestricted_guest || is_paging(vcpu))
>>> 			guest_cr3 = kvm_read_cr3(vcpu);
>>> 		else
>>> 			guest_cr3 = to_kvm_vmx(kvm)->ept_identity_map_addr;
>>> 		ept_load_pdptrs(vcpu);
>>> 	}
>>> 
>>> -	vmcs_writel(GUEST_CR3, guest_cr3);
>>> +	if (!skip_cr3)
>> 
>> Nit: Itâs a matter of taste, but I prefer positive conditions. i.e. âbool
>> write_guest_cr3â.
>> 
>> Anyway, code seems valid to me. Nice catch.
>> Reviewed-by: Liran Alon <liran.alon@xxxxxxxxxx>
>> 
>> -Liran
>> 
>>> +		vmcs_writel(GUEST_CR3, guest_cr3);
>>> }
>>> 
>>> int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>>> -- 
>>> 2.22.0
>>> 
>>