Re: [Qemu-devel] [QEMU PATCH v3 8/9] KVM: i386: Add support for KVM_CAP_EXCEPTION_PAYLOAD

[Liran Alon]

> On 18 Jun 2019, at 12:07, Dr. David Alan Gilbert <address@hidden> wrote:
> 
> * Liran Alon (address@hidden) wrote:
>> Kernel commit c4f55198c7c2 ("kvm: x86: Introduce KVM_CAP_EXCEPTION_PAYLOAD")
>> introduced a new KVM capability which allows userspace to correctly
>> distinguish between pending and injected exceptions.
>> 
>> This distinguish is important in case of nested virtualization scenarios
>> because a L2 pending exception can still be intercepted by the L1 hypervisor
>> while a L2 injected exception cannot.
>> 
>> Furthermore, when an exception is attempted to be injected by QEMU,
>> QEMU should specify the exception payload (CR2 in case of #PF or
>> DR6 in case of #DB) instead of having the payload already delivered in
>> the respective vCPU register. Because in case exception is injected to
>> L2 guest and is intercepted by L1 hypervisor, then payload needs to be
>> reported to L1 intercept (VMExit handler) while still preserving
>> respective vCPU register unchanged.
>> 
>> This commit adds support for QEMU to properly utilise this new KVM
>> capability (KVM_CAP_EXCEPTION_PAYLOAD).
> 
> Does this kvm capability become a requirement for the nested migration
> then? If so, is it wired into the blockers?
> 
> Dave
> 

That’s a very good point.
Yes this capability is required in order to correctly migrate VMs running 
nested hypervisors.
I agree that I should add a migration blocker for nested in case it isn’t 
present.
Nice catch.

-Liran