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SubjectRe: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST_CR3 during nested VM-Enter
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> On 27 Sep 2019, at 17:27, Sean Christopherson <sean.j.christopherson@intel.com> 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@intel.com> 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 “If an emulated VMEntry is squashed because L1 sets
>> vmcs12->guest_activity_state to HLT”. I think it’s 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@vger.kernel.org
>>> Reported-by: Reto Buerki <reet@codelabs.ch>
>>> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
>>> ---
>>> 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, it’s not exactly if L2 is entering HLT state in
>> general. (E.g. issue doesn’t occur if L2 runs HLT directly which is not
>> configured to be intercepted by vmcs12). It’s 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 isn’t related.
So I would explicitly mention it’s when L1 enters L2 but don’t 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: It’s a matter of taste, but I prefer positive conditions. i.e. “bool
>> write_guest_cr3”.
>>
>> Anyway, code seems valid to me. Nice catch.
>> Reviewed-by: Liran Alon <liran.alon@oracle.com>
>>
>> -Liran
>>
>>> + vmcs_writel(GUEST_CR3, guest_cr3);
>>> }
>>>
>>> int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
>>> --
>>> 2.22.0
>>>
>>

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