Re: 2 CPU Conformance Issue in KVM/x86

Avi Kivity
2015-03-09T18:23:12Z

On 03/09/2015 07:51 PM, Nadav Amit wrote:
> Avi Kivity <avi.kivity@gmail.com> wrote:
>
>> On 03/03/2015 11:52 AM, Paolo Bonzini wrote:
>>>> In this
>>>> case, the VM might expect exceptions when PTE bits which are higher than the
>>>> maximum (reported) address width are set, and it would not get such
>>>> exceptions. This problem can easily be experienced by small change to the
>>>> existing KVM unit-tests.
>>>>
>>>> There are many variants to this problem, and the only solution which I
>>>> consider complete is to report to the VM the maximum (52) physical address
>>>> width to the VM, configure the VM to exit on #PF with reserved-bit
>>>> error-codes, and then emulate these faulting instructions.
>>> Not even that would be a definitive solution. If the guest tries to map
>>> RAM (e.g. a PCI BAR that is backed by RAM) above the host MAXPHYADDR,
>>> you would get EPT misconfiguration vmexits.
>>>
>>> I think there is no way to emulate physical address width correctly,
>>> except by disabling EPT.
>> Is the issue emulating a higher MAXPHYADDR on the guest than is available
>> on the host? I don't think there's any need to support that.
>>
>> Emulating a lower setting on the guest than is available on the host is, I
>> think, desirable. Whether it would work depends on the relative priority
>> of EPT misconfiguration exits vs. page table permission faults.
> Thanks for the feedback.
>
> Guest page-table permissions faults got priority over EPT misconfiguration.
> KVM can even be set to trap page-table permission faults, at least in VT-x.
> Anyhow, I don’t think it is enough.

Why is it not enough? If you trap a permission fault, you can inject any
exception error code you like.

> Here is an example
>
> My machine has MAXPHYADDR of 46. I modified kvm-unit-tests access test to
> set pte.45 instead of pte.51, which from the VM point-of-view should cause
> the #PF error-code indicate the reserved bits are set (just as pte.51 does).
> Here is one error from the log:
>
> test pte.p pte.45 pde.p user: FAIL: error code 5 expected d
> Dump mapping: address: 123400000000
> ------L4: 304b007
> ------L3: 304c007
> ------L2: 304d001
> ------L1: 200002000001

This is with an ept misconfig programmed into that address, yes?

> As you can see, the #PF should have had two reasons: reserved bits, and user
> access to supervisor only page. The error-code however does not indicate the
> reserved-bits are set.
>
> Note that KVM did not trap any exit on that faulting instruction, as
> otherwise it would try to emulate the instruction and assuming it is
> supported (and that the #PF was not on an instruction fetch), should be able
> to emulate the #PF correctly.
> [ The test actually crashes soon after this error due to these reasons. ]
>
> Anyhow, that is the reason for me to assume that having the maximum
> MAXPHYADDR is better.
>

Well, that doesn't work for the reasons Paolo noted. The guest can have
a ivshmem device attached, and map it above a host-supported virtual
address, and suddenly it goes slow.