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

Jim Mattson
2017-12-01T23:00:23Z

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.

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.