Re: [Qemu-devel] [PATCH v1 4/5] kvm/x86: Hyper-V VMBus hypercall userspace exit

['Roman Kagan']

On Thu, Jan 14, 2016 at 11:30:43AM +0300, Pavel Fedin wrote:
> > --- a/Documentation/virtual/kvm/api.txt
> > +++ b/Documentation/virtual/kvm/api.txt
> > @@ -3359,6 +3359,14 @@ Hyper-V SynIC state change. Notification is used to 
> > remap SynIC
> >  event/message pages and to enable/disable SynIC messages/events processing
> >  in userspace.
> > 
> > +           /* KVM_EXIT_HYPERV_HCALL */
> > +           struct {
> > +                   __u64 input;
> > +                   __u64 params[2];
> > +                   __u64 result;
> > +           } hv_hcall;
> > +Indicates that the VCPU exits into userspace to process some guest
> > +Hyper-V hypercalls.
> 
>  Why introducing this? We already have KVM_EXIT_HYPERCALL, which is exactly 
> the same. AFAIK it's not used at the moment.
>  Additionally, in theory we could have hypercalls handled in userspace for 
> something else except HyperV. And not only for x86.

We thought reusing KVM_EXIT_HYPERCALL was a bad idea exactly because of
that.  Hypercalls are not universal, the calling and return conventions
are hypervisor-specific.  KVM already has to make the decision that the
particular vmexit is a HyperV hypercall; it appears unnatural to then
pass the data on to userspace in a generic structure and have them make
that decision again.

Roman.