I want a user process in guest machine call a custom hypercall and qemu receives it. I don't want to make any modification of a guest kernel.
From this answer and other materials, I know that vmcall instruction will cause VMEXIT and VMM will receive the its exit reason and arguments.
According to Intel® 64 and IA-32 Architectures Software Developer’s Manual p.1201, vmcall instruction will trigger an exception when CPL > 0.
So I conclude that I need a (guest) kernel interface to invoke a hypercall.
I found that arch/x86/include/asm/kvm_para.h in Linux kernel has kvm_hypercallx functions (where x is a number of arguments). But I can't find a call site of these functions.
Is it possible to invoke a hypercall without any modification of a guest kernel? If so, how to do it? If not, is there any alternative?