Re: [Qemu-devel] [PATCH v4 5/5] kvm/x86: Hyper-V kvm exit

[Pavel Fedin]

 Hello!

> >   It depends. Can i read about these hypercalls somewhere? Is there any 
> > documentation?
> I don't know about a documentation, but you can look at the code of
> Hyper-V hypercall handling inside KVM:
> 
> https://github.com/torvalds/linux/blob/master/arch/x86/kvm/hyperv.c#L346

 Aha, i see, so vmmcall CPU instruction is employed. Well, i believe this very 
well fits into the sematics of KVM_EXIT_HYPERCALL,
because it's a true hypercall.

> The code simply decodes hypercall parameters from vcpu registers then
> handle hypercall code in switch and encode return code inside vcpu
> registers. Probably encode and decode of hypercall parameters/return
> code can be done in QEMU so we need only some exit with parameter that
> this is Hyper-V hypercall and probably KVM_EXIT_HYPERCALL is good for it.

 Or you could even reuse the whole structure, it has all you need:

                        __u64 nr;               /* Reserved for x86, other 
architectures can use it, for example ARM "hvc #nr" */
                        __u64 args[6];  /* rax, rbx, rcx, rdx, rdi, rsi */
                        __u64 ret;
                        __u32 longmode; /* longmode; other architectures (like 
ARM64) can also make sense of it */

 Or you could put in struct kvm_regs instead of args and ret, and allow the 
userspace to manipulate it.

> But KVM_EXIT_HYPERCALL is not used inside KVM/QEMU so requires
> implementation.

 I guess your hypercalls to be introduced using KVM_EXIT_HYPERV are also not 
used inside qemu so require implementation :)

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