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

[Pavel Fedin]

 Hello!

> 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.

 Treatment of them is hypervisor-specific, but from CPUs point of view they are 
the same. You load something into registers, and
execute hypercall instruction. So, you just need to pass registers in your 
structure. Or, you could even use generic register access
APIs.

>  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.

 Is it so difficult to make such a decision? The userland already knows what we 
are emulating.
 I'm afraid that in future we can end up in having 10 versions of 
KVM_EXIT_xxx_HYPERCALL with very small difference between them.
Will it be good?

Kind regards,
Pavel Fedin
Senior Engineer
Samsung Electronics Research center Russia