linux-kernel - Re: [PATCH v4 3/3] KVM: nVMX: Emulate EPTP switching for the L1 hypervisor

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Date:   Tue, 11 Jul 2017 16:34:04 -0400
From:   Bandan Das <bsd@...hat.com>
To:     Radim Krčmář <rkrcmar@...hat.com>
Cc:     David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org,
        pbonzini@...hat.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 3/3] KVM: nVMX: Emulate EPTP switching for the L1 hypervisor

Radim Krčmář <rkrcmar@...hat.com> writes:

> 2017-07-11 15:50-0400, Bandan Das:
>> Radim Krčmář <rkrcmar@...hat.com> writes:
>> > 2017-07-11 14:24-0400, Bandan Das:
>> >> Bandan Das <bsd@...hat.com> writes:
>> >> > If there's a triple fault, I think it's a good idea to inject it
>> >> > back. Basically, there's no need to take care of damage control
>> >> > that L1 is intentionally doing.
>> >> >
>> >> >>> +			goto fail;
>> >> >>> +		kvm_mmu_unload(vcpu);
>> >> >>> +		vmcs12->ept_pointer = address;
>> >> >>> +		kvm_mmu_reload(vcpu);
>> >> >>
>> >> >> I was thinking about something like this:
>> >> >>
>> >> >> kvm_mmu_unload(vcpu);
>> >> >> old = vmcs12->ept_pointer;
>> >> >> vmcs12->ept_pointer = address;
>> >> >> if (kvm_mmu_reload(vcpu)) {
>> >> >> 	/* pointer invalid, restore previous state */
>> >> >> 	kvm_clear_request(KVM_REQ_TRIPLE_FAULT, vcpu);
>> >> >> 	vmcs12->ept_pointer = old;
>> >> >> 	kvm_mmu_reload(vcpu);
>> >> >> 	goto fail;
>> >> >> }
>> >> >>
>> >> >> The you can inherit the checks from mmu_check_root().
>> >> 
>> >> Actually, thinking about this a bit more, I agree with you. Any fault
>> >> with a vmfunc operation should end with a vmfunc vmexit, so this
>> >> is a good thing to have. Thank you for this idea! :)
>> >
>> > SDM says
>> >
>> >   IF tent_EPTP is not a valid EPTP value (would cause VM entry to fail
>> >   if in EPTP) THEN VMexit;
>> 
>> This section here:
>> As noted in Section 25.5.5.2, an execution of the
>> EPTP-switching VM function that causes a VM exit (as specified
>> above), uses the basic exit reason 59, indicating “VMFUNC”.
>> The length of the VMFUNC instruction is saved into the
>> VM-exit instruction-length field. No additional VM-exit
>> information is provided.
>> 
>> Although, it adds (as specified above), from testing, any vmexit that
>> happens as a result of the execution of the vmfunc instruction always
>> has exit reason 59.
>> 
>> IMO, the case David pointed out comes under "as a result of the
>> execution of the vmfunc instruction", so I would prefer exiting
>> with reason 59.
>
> Right, the exit reason is 59 for reasons that trigger a VM exit
> (i.e. invalid EPTP value, the four below), but kvm_mmu_reload() checks
> unrelated stuff.
>
> If the EPTP value is correct, then the switch should succeed.
> If the EPTP is correct, but bogus, then the guest should get
> EPT_MISCONFIG VM exit on its first access (when reading the
> instruction).  Source: I added

My point is that we are using kvm_mmu_reload() to emulate eptp
switching. If that emulation of vmfunc fails, it should exit with reason
59.

>   vmcs_write64(EPT_POINTER, vmcs_read64(EPT_POINTER) | (1ULL << 40));
>
> shortly before a VMLAUNCH on L0. :)

What happens if this ept pointer is actually in the eptp list and the guest
switches to it using vmfunc ? I think it will exit with reason 59.

> I think that we might be emulating this case incorrectly and throwing
> triple faults when it should be VM exits in vcpu_run().

No, I agree with not throwing a triple fault. We should clear it out.
But we should emulate a vmfunc vmexit back to L1 when kvm_mmu_load fails.

>> > and no other mentions of a VM exit, so I think that the VM exit happens
>> > only under these conditions:
>> >
>> >   — The EPT memory type (bits 2:0) must be a value supported by the
>> >     processor as indicated in the IA32_VMX_EPT_VPID_CAP MSR (see
>> >     Appendix A.10).
>> >   — Bits 5:3 (1 less than the EPT page-walk length) must be 3, indicating
>> >     an EPT page-walk length of 4; see Section 28.2.2.
>> >   — Bit 6 (enable bit for accessed and dirty flags for EPT) must be 0 if
>> >     bit 21 of the IA32_VMX_EPT_VPID_CAP MSR (see Appendix A.10) is read
>> >     as 0, indicating that the processor does not support accessed and
>> >     dirty flags for EPT.
>> >   — Reserved bits 11:7 and 63:N (where N is the processor’s
>> >     physical-address width) must all be 0.
>> >
>> > And it looks like we need parts of nested_ept_init_mmu_context() to
>> > properly handle VMX_EPT_AD_ENABLE_BIT.
>> 
>> I completely ignored AD and the #VE sections. I will add a TODO item
>> in the comment section.
>
> AFAIK, we don't support #VE, but AD would be nice to handle from the

Nevertheless, it's good to have the nested hypervisor be able to use it
just like vmfunc.

> beginning.  (I think that caling nested_ept_init_mmu_context() as-is
> isn't that bad.)

Ok, I will take a look.