Re: [PATCH v6 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async_pf

["Radim Krčmář" <rkrcmar@redhat.com>]

2017-06-28 14:56+0200, Paolo Bonzini:
> On 28/06/2017 14:25, Wanpeng Li wrote:
>> From: Wanpeng Li <wanpeng.li@hotmail.com>
>> 
>> Add an nested_apf field to vcpu->arch.exception to identify an async page 
>> fault, and constructs the expected vm-exit information fields. Force a 
>> nested VM exit from nested_vmx_check_exception() if the injected #PF is 
>> async page fault. Extending the userspace interface KVM_GET_VCPU_EVENTS
>> and KVM_SET_VCPU_EVENTS for live migration.
>> 
>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>> Cc: Radim Krčmář <rkrcmar@redhat.com>
>> Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
>> ---
> 
> Radim, Wanpeng,
> 
> the patch is nice now but I'm still not 100% sure about the live
> migration part.  Why do we need to pass nested_apf to userspace, but not
> nested_apf_token?

We do not need it for migration, but unavailable nested_apf_token
already breaks checkpoint & restore from userspace ... I think the
cleanest way would be to add a new paravirtual event for nested_apf.
(Or just keep delaying the apf.)

Migration does a "async-pf-broadcast" while setting the async-pf MSR on
destination, which resumes all async-pf waiters.
Userspace actually has to drop the async-pf event on migration, because
the destination has invalid nested_apf_token.  (It's a horrible design.)

nested_apf is not #PF: if we didn't pass nested_apf, then the exception
would be injected as #PF to L2 after migration.  (Local KVM could
remember that the #PF is nested_apf and do some ugly hacks.)