Re: [PATCH v3 8/8] KVM: x86: avoid loading PDPTRs after migration when possible

[Sean Christopherson <seanjc@google.com>]

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 11260e83518f..eadfc9caf500 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -815,6 +815,8 @@ int load_pdptrs(struct kvm_vcpu *vcpu, struct kvm_mmu *mmu, unsigned long cr3)
>  
>  	memcpy(mmu->pdptrs, pdpte, sizeof(mmu->pdptrs));
>  	kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
> +	vcpu->arch.pdptrs_restored_oob = false;
> +
>  out:
>  
>  	return ret;
> @@ -10113,6 +10115,7 @@ static int __set_sregs2(struct kvm_vcpu *vcpu, struct kvm_sregs2 *sregs2)
>  
>  		kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
>  		mmu_reset_needed = 1;
> +		vcpu->arch.pdptrs_restored_oob = true;

Setting pdptrs_restored_oob[*] here and _only_ clearing it on successful
load_pdptrs() is not robust.  Potential problems once the flag is set:

  1.  Userspace calls KVM_SET_SREGS{,2} without valid PDPTRs.  Flag is now stale.
  2.  kvm_check_nested_events() VM-Exits to L1 before the flag is processed.
      Flag is now stale.

(2) might not be problematic in practice since the "normal" load_pdptrs()
should reset the flag on the next VM-Enter, but it's really, really hard to tell.
E.g. what if an SMI causes an exit and _that_ non-VM-Enter reload of L2 state
is the first to trip the flag?  The bool is essentially an extension of
KVM_REQ_GET_NESTED_STATE_PAGES, I think it makes sense to clear the flag whenever
KVM_REQ_GET_NESTED_STATE_PAGES is cleared.

Another thing that's not obvious is the required ordering between KVM_SET_SREGS2
and KVM_SET_NESTED_STATE.  AFAICT it's not documented, but that may be PEBKAC on
my end.  E.g. what happens if walk_mmu == &root_mmu (L1 active in targte KVM)
when SET_SREGS2 is called, and _then_ KVM_SET_NESTED_STATE is called?

[*] pdptrs_from_userspace in Paolo's tree.