Re: [PATCH v2] KVM: nVMX/nSVM: Don't intercept #UD when running L2

Jim Mattson
2018-01-10T19:56:12Z

Liran, Are you planning to submit reverts for ae1f57670703 and ac9b305caa0d?

On Fri, Dec 1, 2017 at 4:27 PM, Liran Alon <LIRAN.ALON@oracle.com> wrote:
>
>
> On 02/12/17 01:00, Jim Mattson wrote:
>>
>> How does this change interact with commit 84cffe499b94 ("kvm: Emulate
>> MOVBE")?
>>
>> If the hardware doesn't support MOVBE, but L0 sets CPUID.01H:ECX.MOVBE
>> in L1's emulated CPUID information, then L1 is likely to pass that
>> CPUID bit through to L2. L2 will expect MOVBE to work, but if L1
>> doesn't intercept #UD, then any MOVBE instruction executed in L2 will
>> raise #UD, and the exception will be delivered in L2.
>>
>
> Nice catch.
>
> When I considered the functionality of the original commit which I attempted
> to fix (commit ae1f57670703 ("KVM: nVMX: Do not emulate #UD while in guest
> mode")), my patch made sense as a more complete and better fix.
>
> However, both my patch and the original patch don't seem to consider the
> issue you present here. I completely agree.
>
> Maybe it was better just reverting my patch and commit ae1f57670703. I think
> the attempt of that patch was to make L0 not simulate behaviour it simulates
> for L1 in L2 as-well. but after reading your reply, I think that it is a
> desired behaviour...
>
> -Liran
>
>
>>
>> On Fri, Nov 10, 2017 at 1:37 PM, Radim Krčmář <rkrcmar@redhat.com> wrote:
>>>
>>> 2017-11-06 16:15+0200, Liran Alon:
>>>>
>>>> When running L2, #UD should be intercepted by L1 or just forwarded
>>>> directly to L2. It should not reach L0 x86 emulator.
>>>> Therefore, set intercept for #UD only based on L1 exception-bitmap.
>>>>
>>>> Also add WARN_ON_ONCE() on L0 #UD intercept handlers to make sure
>>>> it is never reached while running L2.
>>>>
>>>> This improves commit ae1f57670703 ("KVM: nVMX: Do not emulate #UD while
>>>> in guest mode") by removing an unnecessary exit from L2 to L0 on #UD
>>>> when L1 doesn't intercept it.
>>>>
>>>> In addition, SVM L0 #UD intercept handler doesn't handle correctly the
>>>> case it is raised from L2. In this case, it should forward the #UD to
>>>> guest instead of x86 emulator. As done in VMX #UD intercept handler.
>>>> This commit fixes this issue as-well.
>>>>
>>>> Signed-off-by: Liran Alon <liran.alon@oracle.com>
>>>> Reviewed-by: Nikita Leshenko <nikita.leshchenko@oracle.com>
>>>> Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>> ---
>>>
>>>
>>> Applied, thanks.